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.

Still, the president’s actions — which come about a month before the midterm elections and could help energize Democratic supporters — represent a fundamental change in America’s response to a drug that has been at the center of a clash between culture and policing for more than a half-century.

“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.