FBI spokeswoman Gina Swankie would not confirm anything about Yee or others who may be targeted by the agency, but said "the FBI is executing multiple search warrants and conducting arrests in multiple locations today."
"At this time we are not elaborating due to the need for agent safety," Swankie said.
Yee's press secretary Dan Lieberman said he had no information but expected to provide a statement later today. Yee was photographed entering the federal building in San Francisco, apparently to be formally charged.
News reports said that a well-known former Chinatown gangster, Raymond "Shrimp Boy" Chow, also had been arrested.
Yee, a child psychologist, has served in the Assembly and the Senate and is now running for secretary of state, with another state senator, Alex Padilla of Los Angeles, his chief Democratic foe. Yee has carried a wide variety of legislation during his career and is best known for his measures dealing with violence in video games and his advocacy for open records.
Two other Democratic state senators, Rod Wright and Ron Calderon, are already on leaves of absence due to criminal charges against them, and their departure erased the Senate's Democratic supermajority at least temporarily. Were Yee also to depart before his term expires, it would drop Democrats to 25 seats in the Senate and give Republicans a larger role in legislation that requires two-thirds votes, such as tax increases, constitutional amendments and a pending water bond issue.
A spokesman for Senate leader Darrell Steinberg said the pro tem planned to talk to media this afternoon. Minority leader Bob Huff issued a statement by email saying he is "deeply troubled" by Yee's arrest.
"Once again, the Senate has been tarnished by another FBI raid of a Senator's capitol office. There are hundreds of visitors in the State Capitol each day, and those who witnessed this morning's events have every reason to be concerned about whether the Legislature is more concerned about serving themselves than the people."
Updated at 11:48 a.m. with additional details.
PHOTO: State Sen. Leland Yee, D-San Francisco, arrives at the San Francisco Federal Building on March 26, 2014 after being arrested on suspicion of corruption charges. Courtesy of KPIX