WASHINGTON — President Trump has pardoned, commuted the prison sentences, or vowed to dismiss the cases of all of the 1,500-plus people charged with crimes in the January 6, 2021, Capitol riot, including people convicted of assaulting police officers.

Mr. Trump’s use of his clemency powers, just hours after his return to the White House on Monday, paves the way for the release from prison of people found guilty of violent attacks on police, as well as leaders of right-wing extremist groups convicted of failed plots to keep the Republican in power after he lost the 2020 election to President Biden.

The pardons are a culmination of Mr. Trump’s yearslong campaign to rewrite the history of the January 6 attack, which left more than 100 police officers injured as the angry mob of Trump supporters — some armed with poles, bats, and bear spray — overwhelmed law enforcement, shattered windows, and sent lawmakers and aides running into hiding.

While pardons were expected, the speed and the scope of the clemency amounted to a stunning dismantling of the Justice Department’s effort to prosecute what it called “crimes committed at the U.S. Capitol” on January 6.

Mr. Trump also ordered the attorney general to seek the dismissal of roughly 450 cases that are pending before judges stemming from the largest investigation in Justice Department history.

Casting the rioters as “patriots” and “hostages,” Mr. Trump has claimed they were unfairly treated by the Justice Department, which also charged him with federal crimes in two cases he contends were politically motivated.

Mr. Trump said the pardons will end “a grave national injustice that has been perpetrated upon the American people over the last four years” and begin “a process of national reconciliation.”

The pardons were met with elation from Trump supporters and lawyers for the January 6 defendants. Backers of the president gathered late Monday in the cold outside the Washington jail, where more than a dozen defendants were being held before the pardons.

“We are deeply thankful for President Trump for his actions today,” attorney James Lee Bright said, who represented Oath Keepers founder Stewart Rhodes, who was serving an 18-year prison sentence after being convicted of seditious conspiracy and other crimes.

It’s unclear how quickly the defendants may be released from prison. An attorney for the former Proud Boys national chairman, Enrique Tarrio, who was sentenced to 22 years in prison for seditious conspiracy, said he expected his client to be released from prison Monday night.

“This marks a pivotal moment in our client’s life, and it symbolizes a turning point for our nation,” attorney Nayib Hassan said in a statement. “We are optimistic for the future, as we now turn the page on this chapter, embracing new possibilities and opportunities.”

Democrats slammed the move to extend the pardons to violent rioters, many of whose crimes were captured on camera and broadcast on live TV.

Speaker Pelosi called it “an outrageous insult to our justice system and the heroes who suffered physical scars and emotional trauma as they protected the Capitol, the Congress and the Constitution.”

“Donald Trump is ushering in a Golden Age for people that break the law and attempt to overthrow the government,” Senator Schumer said in an emailed statement.

A former Metropolitan Police officer, Michael Fanone, who lost consciousness and suffered a heart attack after a rioter shocked him with a stun gun, appeared taken aback to learn from an Associated Press reporter that those who assaulted police officers are among the pardon recipients.

“This is what the American people voted for,” he said. “How do you react to something like that?”

Mr. Fanone said he has spent the past four years worried about his safety and the well-being of his family. Pardoning his assailants only compounds his fears, he said.

“I think they’re cowards,” he said. “Their strength was in their numbers and the mob mentality. And as individuals, they are who they are.”

Mr. Trump had suggested in the weeks leading up to his return to the White House that instead of blanket pardons, he would look at the January 6 defendants on a case-by-case basis.

Vice President Vance had said just days ago that people responsible for the violence during the Capitol riot “obviously” should not be pardoned.

Fourteen defendants, including several convicted of seditious conspiracy, had their sentences commuted, while the rest of those found guilty of January 6 crimes were granted “full, complete and unconditional” pardons.