OAKLAND  (AP) — Federal agents on Monday targeted a San Francisco Bay area medical marijuana training school started by a leading pot advocate who has been instrumental in pushing for ballot measures to legalize the drug.

The doors to Oaksterdam University in downtown Oakland were blocked by U.S. marshals and yellow tape following the early morning raid by agents with the Internal Revenue Service and the U.S. Drug Enforcement Administration.

Agents carted trash bags of unknown materials out of the school as protesters gathered to condemn the action. A museum connected to the school and a nearby medical marijuana dispensary operated by Oaksterdam founder Richard Lee also were raided.

Demonstrators outside the multistory building, some openly smoking marijuana, held signs demanding an end to federal crackdowns on marijuana, which remains illegal under federal law.

Ryan Hooper, 26, of Oakland, wearing an Oaksterdam hat and sweat shirt, said he had finished taking courses at the school in February.

"This is not in the best interest of the city," Hooper said. "If they close the dispensaries, all of this stuff is going to go back underground."

Oaksterdam University was founded by Lee, who spent more than $1 million as the main backer of a California ballot measure defeated in 2010 that would have legalized marijuana in the state for recreational use. Lee did not immediately return a message seeking comment.

The school offers classes to would-be medical marijuana providers in fields ranging from horticulture to business to the legal ins-and-outs of running a dispensary. It does not distribute marijuana.

Arlette Lee, an IRS spokeswoman and no relation to Richard Lee, told reporters that agents were serving a federal search warrant but said she could not otherwise comment on the purpose of the raid.

"What we are doing here today is under seal," Lee said.

Agents also raided Richard Lee's home and briefly detained him during their search but did not arrest him, said Dale Sky Jones, Oaksterdam's executive chancellor.

"Clearly, they're trying to knock down one of the leaders in the cannabis reform movement," Jones said.

No other arrests were reported, and it was unclear if the raid was prompted by a civil or criminal complaint. Jack Gillund, a spokesman for the U.S. attorney's office, also declined comment.

The raid was the latest move by the federal government to crack down on California's thriving medical marijuana industry. Federal prosecutors across the state joined late last year to shut down dozens of dispensaries by threatening to seize landlords' property if they did not evict marijuana retailers.

The government's action came as a surprise to medical marijuana advocates because the city of Oakland has been somewhat of a safe haven for pot clinics. The city has long allowed four medical marijuana dispensaries to legally operate under city ordinances and recently awarded permits that would allow four more to open.

"Oakland has one of the most highly regulated systems for distributing medical marijuana in the state," said Stephen Gutwillig, California's director for the Drug Policy Alliance. "We think this is a campaign by the U.S. attorneys not just to limit but to kill access to medical marijuana in California."

Others countered that pot advocates are mistaken if they believe the Obama administration wouldn't take action.

"This is a warning signal to any city including Oakland that they should tread very carefully when sanctioning an illegal activity," said Kevin Sabet, a former senior adviser to the president's drug czar and an assistant professor at the University of Florida. "The brazenness of Oakland and other cities like this has actually made them a target."

Some observers said the federal government's decision to go after Oaksterdam shows it's not going to back down.

"It doesn't get much more confrontational than that," said Alex Kreit, a law professor at the Thomas Jefferson School of Law in San Diego.