OAKLAND — Federal agents delivered a blow to the heart of the medical cannabis movement early Monday, raiding four Oaksterdam University sites in downtown Oakland and the home of the institution’s founder, Richard Lee.

Authorities refused to provide details about the raids carried out by U.S. Marshals and agents with the Drug Enforcement Agency and Internal Revenue Service. Lee was briefly detained in his home, but not arrested, supporters said. Two protesters were arrested as agents seized marijuana plants and other materials from Oaksterdam’s downtown Oakland locations, all of which remained closed Monday.

The raid comes six months into a federal crackdown on California’s medical marijuana industry. Authorities have forced dozens of dispensaries to close or relocate and threatened to seize the properties of landlords that rent to dispensaries near schools and parks.

Lee, a pioneer in the movement to tax and regulate marijuana and a key figure behind the failed 2010 state ballot measure to legalize the drug, is a leading light in the medical cannabis movement — and consequently an obvious target for federal authorities, supporters say.

“The federal government is trying to send a message that it’s as big a bully as it wants to be, and Richard Lee is a good sacrificial lamb,” said Robert Raich, an attorney and instructor at Oaksterdam, which offers courses for people interested in the marijuana industry.

Lee didn’t return phone calls Monday, but fellow medical marijuana advocate Jeff Jones said Lee was “was upbeat and happy not to be in custody.”

Despite the federal crackdown, Oakland has remained supportive of the medical marijuana industry, which last year generated $1.68 million in tax revenue. In March, the city pressed ahead with permits to double the number of dispensaries from four to eight, and several officials criticized Monday’s raid.

“We have a serious gun violence problem in Oakland,” Councilwoman Rebecca Kaplan said. “If there are extra law enforcement resources available, they should be focused on fighting illegal guns and gun violence.”

City Attorney Barbara Parker said she opposed “federal raids and prosecutions of Oakland businesses that are complying with state and local laws and regulations and paying their fair share of taxes.”

This isn’t the first time Lee has found himself in the cross hairs of federal authorities. He recently was forced to move his Coffeeshop Blue Sky dispensary from 17th Street to the Oaksterdam Cannabis Museum after a school moved near the former site. Authorities also have targeted the much larger Harborside Health Center dispensary in East Oakland, hitting it last year with a $2.4 million bill for back taxes for claiming standard business deductions.

Monday’s raid began about 8 a.m. outside the college at 1600 Broadway. Protesters soon arrived, with their numbers swelling to more than 100 by noon. Authorities also raided the museum at the corner of Broadway and 19th Street, as well as the Oaksterdam Gift Shop at 405 15th Street. Jones said Lee has a second storefront on 15th Street that also was raided.

“They took all his materials, killed all his plants,” Jones said. “The idea was to demoralize the owner and the staff.”

The protests climaxed shortly after 12:30 p.m., when agents carried a safe and dozens of boxes of documents and black garbage bags apparently filled with plants out of the building and into a waiting moving truck.

Protesters then tried to block the DEA’s vehicles from leaving, stopping traffic on Broadway and scuffling with Oakland police officers who arrived on the scene to control the crowd.

Two people were arrested as people swarmed the street. Oakland police arrested one woman on suspicion of delaying or obstructing an officer, and U.S. Marshals arrested one man on suspicion of assaulting a federal officer, officials said.