Former Dallas County Sheriff James G. Clark, who led a mounted posse against black demonstrators at Selma, Ala., in 1965, has been arrested by federal authorities at a Birmingham hospital where he is scheduled for surgery.

Clark and six others were indicted on charges of conspiring to bring marijuana into the United States from Colombia. The indictments charge that with Clark and the others were connected with importing 6,600 pounds of marijuana confiscated by authorities on a plane that had landed at Montgomery earlier this year.

Officials also broke up several drug operations in Florida.

More than $1 million in cash and seven pounds of cocaine were seized and 10 men arrested at a Fort Lauderdale hotel.

Police also said a small airplane was seized in their Monday night operation.

Meanwhile, 23 men were arrested yesterday and more than 37 tons of marijuana was picked up in separate operations by U.S. Customs and the U.S. Coast Guard.

Customs officers waiting in ambush seized two boats loaded with 17 tons of pot, worth an estimated $10 million, and arrested 12 men near Key Largo, authorites said.

Hours later, Coast Guard officers boarded a vessel 360 miles east of Miami, seized 20 tons of pot and took 11 crewmen in custody, spokesman said.