A day after claiming he didn’t mean to suggest that law enforcement officials should shoot people who were part of the unrest in Minnesota, President Trump said on Saturday that the Secret Service had been prepared to sic the “most vicious dogs” on protesters outside the White House gates on Friday night.

“Great job last night at the White House by the U.S. @SecretService,” Mr. Trump tweeted in a string of four posts on Saturday. “They were not only totally professional, but very cool. I was inside, watched every move, and couldn’t have felt more safe.”

He continued that the Secret Service allowed the protesters to “scream & rant as much as they wanted” and only acted when “someone got too frisky or out of line.”

“The front line was replaced with fresh agents, like magic,” he added. “Big crowd, professionally organized, but nobody came close to breaching the fence. If they had they would have been greeted with the most vicious dogs, and most ominous weapons, I have ever seen. That’s when people would have been really badly hurt, at least.”

Mr. Trump claimed that Secret Service agents told him they were clamoring for engagement with the protesters. “We put the young ones on the front line, sir, they love it, and good practice,” he claimed he had been told.

He also appeared to invite his own supporters to amass outside the White House on Saturday to counter the protesters, despite a ban against gatherings of more than 10 people in effect in Washington amid the coronavirus pandemic.

“Tonight, I understand, is MAGA NIGHT AT THE WHITE HOUSE???” he tweeted, using the acronym for his first campaign slogan, “Make America Great Again.” And he tried to paint the protesters as recruited agitators instead of people organically making a statement.

Mr. Trump’s renewed threat of violence against the protesters came as protests erupted in cities across the country.

And it came a day after he tweeted — and then tried to walk back — that “when the looting starts, the shooting starts” in response to protests in Minneapolis against the killing of George Floyd, an unarmed black man, at the hands of a white police officer.

That phrase was used in 1967 by a Miami police chief in reference to crackdowns on young black men. The chief, Walter Headley, accused the men of operating under cover of the civil rights movement and said his police force didn’t mind being accused of police brutality.

Mr. Trump waited nearly 14 hours to try to walk that statement back, claiming he had meant that when people loot, they then sometimes fire guns as well. And in remarks at a late afternoon event at the White House, Mr. Trump conceded that some of the protesters had legitimate grievances.

Mr. Trump traveled to Florida on Saturday for the second time this week to watch SpaceX try to launch a rocket to the International Space Station. The initial launch was scrubbed shortly before it was set to take place on Wednesday.

Talking to reporters as he left the White House, Mr. Trump was asked about his tweet that seemed to invite his own supporters to rally outside the White House. As he often does, Mr. Trump distanced himself from his own statements, saying he was merely asking a question and that he didn’t know if people were coming. He claimed not to be trying to stoke racial strife.

“By the way, they love African-American people, they love black people,” Mr. Trump volunteered, unprompted, describing his own supporters, who are overwhelmingly white.

“MAGA loves the black people,” he added.

He reiterated his condemnation of Minnesota’s governor and the mayor of Minneapolis, both Democrats, painting the matter in starkly political terms as his opposition to “liberal” activists.

“They’ve got to get tougher. They’ve got to get tougher,” Mr. Trump said of the responses to the unrest, which resulted on Thursday night in the burning of the police station where the officers involved in Mr. Floyd’s death worked. “They’ve got to be strong. Honor the memory of George Floyd, honor his memory. They have to get tougher, and by being tougher they will be honoring his memory, but they cannot let that happen.”

He again raised the prospect of sending in the military to quell the unrest.

“We could have troops on the ground very quickly if they ever want our military,” Mr. Trump said.

At Mr. Trump’s behest, the Defense Department ordered the Army to prepare active-duty military police units to deploy from several army bases to Minneapolis. The move does not mean that the units will be activated, but it is a first step toward doing so, officials said.

Mr. Trump has repeatedly criticized protesting groups that are predominantly made up of black people. In the 2016 campaign, he condemned the Black Lives Matter protesters as a “threat.”

Since he became president, he has also denounced protesters critical of his administration as being members of “antifa,” a contraction of the word “antifascist.” The name has become a catch-all for a loose affiliation of radical activists who oppose the far right.

On Saturday, Mr. Trump insisted that the protesters were far-left extremists, a claim that Attorney General William P. Barr echoed on Saturday.

“Groups of outside radicals and agitators are exploiting the situation to pursue their own separate and violent agenda,” Mr. Barr said.

While it is the responsibility of state and local officials to get the rioting under control, Mr. Barr said the F.B.I. and other federal law enforcement would support those efforts. He warned it was a violation of federal law to cross state lines “to incite or participate in violent rioting.” The Department of Justice and F.B.I., he said, would enforce those laws.

On Twitter on Saturday, Mr. Trump also denounced several mainstream news outlets, a day after a CNN reporter was arrested on camera for no apparent reason by police officers in riot gear in Minneapolis, and a female reporter for a local television station was shot with what appeared to be pepper balls by an officer in Louisville, Ky.

“Much more ‘disinformation’ coming out of CNN, MSDNC, @nytimes and @washingtonpost, by far, than coming out of any foreign country, even combined. Fake News is the Enemy of the People!” Mr. Trump wrote.

Julian E. Barnes contributed reporting.

For days, President Trump has been on a rampage against Twitterfor its treatment of him, and it’s easy to see why. Early Friday morning, after a tweet from him about the violence in Minneapolisdeclared, “When the looting starts, the shooting starts,” Twitter dispatched police officers to the White House, who handcuffed Mr. Trump and took him into custody on live television in view of the entire nation.

Oh, sorry, quick fact-check: That did not happen at all. The president remains free and tweeting. Twitter, a private company, remains free to set rules on the use of its service. The president’s flagged tweets — the “shooting” remark and a misleading attackon mail-in voting — remain available to read, the first behind a notice that it violates the service’s rules on glorifying violence, the second with a fact-checking link appended.

The arrest on live TV Friday was of Omar Jimenez, a CNN reporter, and his crew, who were handcuffed and walked off down a ravaged Minneapolis block, where they’d been covering protests and violence after the killing of a black man, George Floyd, in police custody.

The incident, which unfolded over several tense minutes, was brazen and appalling. But at least it served a clarifying purpose. After days of hot air expended insisting on a politician’s “right” to use a private platform without correction, America got to see what an actual offense against the First Amendment looks like.

It looked like world news footage from a police state. Mr. Jimenez, wearing a mask in the midst of the coronavirus pandemic, calmly negotiates with officers, visored and bunched in around the camera. He tells them they are live on the air, and he offers to get out of their way: “Put us back where you want us.”

He’s told, “You’re under arrest.”

He asks why and gets no answer. And he’s walked off, to the stunned play-by-play of the anchors in the CNN studio. (Mr. Jimenez is black and Latino. Notably, given the racial dynamics of the Minneapolis protests, a white CNN reporter also covering the story said he was treated much more politely.)

Then the producer is arrested, then the cameraman, until finally an officer picks up the camera and walks it off, the screen jostling into motion as if we, the audience, were being taken into custody, for getting too close, for seeing too much, for looking at someone the wrong way.

The official explanation for the arrest was that the CNN crew refused to move on police orders, an absurdity given what the world saw and heard live. “I’ve never seen anything like this,” the network anchor John Berman said.

But we have seen things like this, not long ago, if not so flagrantly and shamelessly. Police in Ferguson, Mo., gave a similar rationalization in 2014 for arresting two journalists — ordering one to “Stop videotaping!” as he recorded his arrest — during the unrest after the police shooting of Michael Brown.

In the past, though, the arrest did not happen to journalists who work for a news organization that the president had designated the “enemy of the people.” It did not happen under a president who once retweeted a doctored video that showed him beating on a person with the CNN logo covering his face.

And it did not happen in a week when that president threatened punitive measures against a private social-media platform for suggesting that the misinformation he tweeted was misinformation. The president, it seems, considers his inconvenience to be a violation of freedom, and actual press freedom to be an inconvenience.

Which in the end is the only real connection between Mr. Trump’s claims of oppression and the violation we watched on morning cable TV. Actual censorship happens when a government acts to suppress protected speech, not when a private company sets rules for using its platform.

Just hours before the arrest, Mr. Trump posted his tweet with the “shooting” line, which the Miami police chief Walter Headley used in 1967 to justify crackdowns on civil rights protesters.

And for years, he has used his speech, copious and unfiltered, to argue that the police should have a free hand in dealing with threats, and that among the greatest threats are news outlets like CNN.

By noon on Friday, the president was still freely grousing about Twitter, on Twitter. His account made no mention of the CNN arrests.

That morning, Mr. Jimenez and his crew were released, with an apology from Minnesota’s governor. But the messages had already been sent. The arrest told all media that there are people within law enforcement who now feel empowered enough to shut down coverage of unrest — unrest resulting from police violence — flat out in the open.

And it told American viewers what kind of country they are living in. This country was captured in the final seconds of video by the CNN camera, laid on the concrete, still rolling, the booted feet of police lined up at a 90-degree angle. A country angry, frightened, smoldering and tilted sideways.