Federal agents on Monday raided a San Francisco Bay area medical marijuana training school at the heart of California's pot legalization movement.

The doors to Oaksterdam University in downtown Oakland were cordoned off by yellow tape and blocked by U.S. marshals. Agents from the U.S. Drug Enforcement Administration carted trash bags of unknown materials out of the school to a waiting van.

Arlette Lee, an IRS spokeswoman, told reporters that agents were serving a federal search warrant but said she could not otherwise comment on the purpose of the raid. She said all documents related to the raid were under seal.

About a dozen protesters upset over the raid gathered out front of the multistory building adorned with a large mural that makes the school one of the neighborhood's most visible landmarks. The demonstrators held signs demanding an end to federal crackdowns on medical marijuana.

Oaksterdam University was founded by Richard Lee, the main backer of the California ballot measure defeated in 2010 that would have legalized marijuana in the state for recreational use.

The neighborhood has also been home to several medical marijuana dispensaries, including one founded by Lee.

Lee did not immediately return a message seeking comment.

The city of Oakland has long allowed four medical marijuana dispensaries to legally operate under city ordinances and recently awarded permits that would allow four more to open.

At the same time, federal prosecutors across California have been working for months to shut down dispensaries by threatening to seize landlords' property if they did not evict marijuana retailers.