MOBILE, Ala. (AP) _ The brother of a civil rights activist faces a possible 40 years in jail and a $10,000 fine following his conviction on charges of trying to extort money from white businessmen by promising he could stop a boycott by blacks.

U.S. District Judge Emmett Cox set sentencing of Ronald Diamond for March 28, following his conviction of attempted extortion and conspiracy. The maximum penalty for each charge is 20 years in prison and the fine.

His brother, Glenn, a former official of a civil rights group founded by the Rev. Martin Luther King Jr., was acquitted Saturday of similar charges in a split verdict by the jury, even though an FBI agent testified he was behind the scheme.

Diamond's attorney, Barre Dumas, said he planned an appeal, andChris Clanton, attorney for Glenn Diamond, called the guilty verdict on Ronald ''a great injustice. ''It is a sad state for this country when the government has to resort to tactics of this type to bring down leaders of our community,'' he said.

Ronald Diamond was convicted of trying to extort money from white-owned businesses in the town of Mount Vernon, 20 miles north of Mobile. The firms were boycotted by the Southern Christian Leadership Conference in a dispute.

FBI agent Jeff Steuer testified Friday that Ronald Diamond was handed $1,500 in an extortion payment Oct. 22.

He said both Diamond brothers were involved in the plot to make white businessmen pay up, telling them the boycott would stop if they did.

But Steur said Glenn Diamond, a vice president of the Mobile County chapter of the SCLC at the time, was ''calling the shots.'' He was removed from office when he was charged.

Ronald Diamond told the court he took the money because he was owed it by the businessman involved.

The SCLC, an Atlanta-based civil rights organization, was founded by King in 1958. It has been prominent in the fight for black voting rights.