Byron De La Beckwith, 56, twice tried but never convicted of the murder in 1963 of Mississippi civil rights leader Medgar W. Evers, was arrested yesterday on a Louisiana fugitive warrant in connection with another case.

Beckwith, a resident of Cruger, Miss., apparently came to Washington to promote an oil filtering device for which he has a franchise. He called on the office of Sen. James O. Eastland (D-Miss.) for assistance.

Courtney Pace, Eastland's administrative assistant, said Beckwith went to the senator's office last Monday "with a gadget he said was an oil filter that would save oil; he wanted to see somebody in this energy business."

Pace said a member of Eastland's staff called the office of James R. Schlesinger, President Carter's chief energy adviser, to find out the location of the "gadget division" of the Energy Research and Development Adminstration.

"We got him the room number and we sent him over there; we haven't seen nor heard from him since," Pace said.

He said Beckwith was treated simply as a constituent and that he was unaware of his legal troubles in Louisiana.

According to D.C. police and Superior Court records, Beckwith was convicted in the Orleans Parish District Court in New Orleans on Aug. 1, 1975 of a charge of transporting explosives on a state road. Judge Charles Ward sentenced him to five years in prison.

Beckwith was released on $10,000 bond pending an appeal. The appeal was denied last Feb. 28. When Beckwith failed to appear to begin serving his sentence, Judge Ward issued a warrant for his arrest.

Lt. Raymond Comstock of the New Orleans police said he had received word that Beckwith might be visiting friends here, D.C. police were alerted and Beckwith was arrested in a house in the 2200 block of Observatory Place NW at 2:50 yesterday morning.

He appeared before Judge William E. Stewart of D.C Superior Court yesterday afternoon and waived his right to an extradition proceeding. Steward ordered him to D.C jail pending the scheduled arrival here today of Louisiana officials who were to return him to New Orleans.

Lt. Comstock said Beckwith was stopped in Louisiana on Interstate Highway 10 after officers received information that he was transporting explosives into the state. He said officers found dynamite, primers, a clock device rigged to set the explosives off, a rifle, a pistol, ammunition and the barrel of a machine gun in Beckwith's car. That incident occurred in September, 1973.

That was 10 years and three months after Medgar Evers, the Mississippi field secretary of the NAACP, was shot to death from ambush in the driveway of his home in Jackson, Miss.

Beckwith was tried twice for the killing. Both court proceedings ended when mistrials were declared, and Beckwith went free.

A D.C. Bail Agency report in Beckwith's Superior Court file said he had given Sen. Eastland as a reference.

The Bail Agency report also said Beckwith had arrived in Washington a week ago, that he had been employed as a salesman by the Southern Resources Co. of Little Rock, Ark., for the past seven months, that he works on commission, and that he had made no money since July, 1976.

Beckwith, a farm machinery salesman for several years, said nothing during his brief court appearance yesterday. He was neatly dressed in a light-blue suit with shirt and tie. He smiled and nodded to a woman sitting among the spectators.

The woman, who declined to give her name, said she was "horrified" when officers arrived at her home yesterday to arrest Beckwith.