WASHINGTON — President Biden on Thursday pardoned thousands of people convicted of marijuana possession under federal law and said his administration would review whether marijuana should still be in the same legal category as drugs like heroin and LSD.

The pardons will clear everyone convicted on federal charges of simple possession since it became a crime in the 1970s. Officials said full data was not available but noted that about 6,500 people were convicted of simple possession between 1992 and 2021, not counting legal permanent residents. The pardons will also affect people who were convicted under District of Columbia drug laws; officials estimated that number to be in the thousands.

The pardons will not apply to people convicted of selling or distributing marijuana. And officials said there are no people now serving time in federal prisons solely for marijuana possession. But the move will help remove obstacles for people trying to get a job, find housing, apply to college or get federal benefits.

Mr. Biden urged governors to follow his lead for people convicted on state charges of simple possession, who vastly outnumber those charged under federal laws.

“Sending people to jail for possessing marijuana has upended too many lives — for conduct that is legal in many states,” Mr. Biden said on Twitter on Thursday. “That’s before you address the clear racial disparities around prosecution and conviction. Today, we begin to right these wrongs.”

In a video, he added: “While white and Black and brown people use marijuana at similar rates, Black and brown people are arrested, prosecuted and convicted at disproportionately higher rates.”

Mr. Biden stopped short of calling for the complete decriminalization of marijuana, which is something that Congress would have to do. But he said on Twitter that the federal government still needs “important limitations on trafficking, marketing and underage sales of marijuana.”

The actions were part of a long evolution on criminal justice for Mr. Biden, who helped pass a string of laws during his 36 years in the Senate that laid the groundwork for mass incarceration. He apologized on the campaign trail for portions of one of the more aggressive measures he had championed, the 1994 crime bill, and he campaigned on providing more leniency to nonviolent drug offenders.

The pardons move the federal government more in line with the positions taken by some state governments, which have already reduced or eliminated the criminal punishments for simply possessing marijuana — punishments that for decades have sent people to prison.

Mr. Biden also said Thursday that he has asked the attorney general to review how marijuana is legally categorized, which helps determine what kind of penalties are involved.

“The federal government currently classifies marijuana as a Schedule 1 substance,” he said, “the same as heroin and LSD and more serious than fentanyl. It makes no sense.”

Some of the president’s Republican critics lashed out at him after the announcement.

“In the midst of a crime wave and on the brink of a recession, Joe Biden is giving blanket pardons to drug offenders,” said Senator Tom Cotton, Republican of Arkansas. “This is a desperate attempt to distract from failed leadership.”

Advocacy groups, including those representing minorities, have been urging Mr. Biden to take action as a way of demonstrating his commitment to reforming the inequities built into the criminal justice system.

Inimai Chettiar, the federal director of the Justice Action Network, called the president’s move “a really good step” and said one of the most important parts of Mr. Biden’s policy is the directive to review how future marijuana crimes are prosecuted.

“That’s trying to change a policy decision that was made that marijuana is as dangerous as these other drugs, which we know is not true,” Ms. Chettiar said.

Udi Ofer, a Princeton University professor and former deputy national political director of the American Civil Liberties Union, said simple possession of marijuana is a crime “almost entirely prosecuted by the states.” The federal government tends to prosecute marijuana trafficking crimes, he said.

Only 92 people were sentenced on federal marijuana possession charges in 2017, out of nearly 20,000 drug convictions, according to the U.S. Sentencing Commission.

“This is an important political statement, it’s an important value statement, it’s progress, but this is a drop in the ocean of injustice,” Mr. Ofer said.

Marijuana is already fully legal in about 20 states, and some other states have relaxed criminal penalties, according to DISA, a large drug-testing company that tracks state laws regarding marijuana. It remains fully illegal in a handful of states. The federal government will stop charging anyone with simple possession starting on Thursday, officials said.

Mr. Biden’s announcement could give Democrats a boost in the upcoming midterm elections, especially among young people, liberals and minority communities.

In July, a half-dozen of the Senate’s most liberal senators wrote Mr. Biden a letter urging him to take the steps he announced on Thursday.

“The administration’s failure to coordinate a timely review of its cannabis policy is harming thousands of Americans, slowing research, and depriving Americans of their ability to use marijuana for medical or other purposes,” wrote the group of senators, including his onetime rivals, Bernie Sanders, independent of Vermont, and Elizabeth Warren, Democrat of Massachusetts.

More recently, John Fetterman, the Democratic candidate for Senate in Pennsylvania, urged the president to act.

“It’s long past time that we finally decriminalize marijuana,” he said on Twitter, adding, “@POTUS you have the power to use your executive authority to chart a new course.”

Some opponents of full marijuana legalization praised Mr. Biden’s move, saying it was a good way to avoid going further.

“No one deserves to be in jail for a joint,” said Kevin Sabet, who leads Smart Approaches to Marijuana, which opposes legalization. “But we should also not be selling highly potent THC products, nor should we promote and encourage use among young people.”

Some criminal justice activists have criticized Mr. Biden for taking too long to enact more lenient sentencing reform proposals like the one he announced on Thursday.

They argue that Mr. Biden was cowed by Republican attacks that blamed the president’s policies for the rise in violent crimes in some parts of the country. Mr. Biden has rejected the call from some members of his party to “defund” the police and has insisted that police need more money to do their jobs.

White House officials and the Domestic Policy Council, led by Susan Rice, also held calls with criminal justice advocates near the end of last year to field ideas for using executive action to enact prison reform. In April, Mr. Biden used clemency to commute the sentences of 75 nonviolent drug offenders.

While studies show white and Black people use marijuana at similar rates, a Black person is more than three times as likely to be arrested for possession than a white person, according to a report from the A.C.L.U. that analyzed marijuana arrest data from 2010 to 2018.

A vast majority of marijuana arrests fall under the jurisdiction of states, but the crime has historically represented about a third of nationwide drug possession arrests by state and federal officials. According to preliminary F.B.I. data, more than 170,800 of the roughly 490,000 drug possession arrests in 2021 were related to marijuana possession.