Federal agents hit Hells Angels headquarters and clubhouses in Los Angeles, San Francisco, Arizona, Washington state, Nevada and Alaska in pre-dawn raids.
Forty arrests were made by federal officials, and local police arrested another 17 in California on state charges. It was not immediately clear how many people were arrested on local charges in other states.
The investigation included law enforcement agents who offered to buy explosives from members of the motorcycle gang, Donald R. Kincaid, special agent in charge of the Los Angeles office of the Bureau of Alcohol, Tobacco, Firearms and Explosives, said at a news conference.
"We purchased some stolen military explosives," he said, adding that they were identified as military property by lot numbers and other distinguishing marks.
Items seized in Los Angeles included about 50 firearms and a quarter-pound of suspected methamphetamine, bureau officials said.
At the news conference, agents displayed motorcycle jackets, memorabilia and rifles, shotguns, semiautomatic weapons and pistols seized from the clubhouse.
Helmeted ATF agents raided the Hells Angels headquarters in Sparks, Nevada, making one arrest and confiscating several items. One member of the club told KRNV-TV the agents took mostly clothing and trophies.
Seven arrests in Arizona included the president of the group's Tucson chapter, the former president of the Mesa chapter and other chapter leaders, authorities said.
In Washington state, there were five arrests for drug trafficking, weapons charges, possession of stolen explosives and other crimes, authorities said. Four arrests were made in Alaska.