Los Angeles police have arrested a teen-ager on suspicion of murder in Thursday's assassination of Kemal Arikan, the Turkish consul general here. A second suspected assailant is being sought.

Harry Sassounian, 19, and three other persons were taken into custody hours after the shooting. The others were released after questioning. Sassounian is expected to be arraigned Monday.

A Los Angeles police special weapons and tactics (SWAT) team conducted a 90-minute search of a Pasadena house to which they traced the suspects, but investigators refused to disclose whether that search had yielded evidence linking Sassounian with the Justice Commandos of Armenian Genocide, an Armenian terrorist group that claimed responsibility for Arikan's murder.

Police were led to Sassounian through a license number given to them by an unidentified eyewitness. The eyewitness, a lawyer, told police that he saw two assailants running away from the corner where Arikan was shot as he sat in his car at a stoplight.

The lawyer said he followed the two in his car until they jumped into a white Camaro parked, with its doors open, about two blocks from the corner where Arikan was shot.

Los Angeles Police Chief Daryl F. Gates said late Thursday that his department had believed Arikan to be in danger and last week had offered him protection, which he said Arikan refused.

Uniformed police have been in evidence at many diplomatic locations around the city, including the Turkish consulate, since Thursday's shooting. Police spokesmen declined to comment on plans for increased protection for other Turkish officials.

When Arikan was killed he was on his way to meet a friend, Akif Keskin. On Thursday Keskin stood behind police lines at the murder site, fighting back tears as he talked to reporters about Arikan, whom he described as a friend "closer than my brother."

"I don't understand," Keskin said. "He was a nice man, one of the nicest . . . and he's never talked against the Armenians, never."

Keskin said that Arikan had spoken to him in the last week about being in some danger: "He knew something like this could happen. But he was never afraid."

Keskin said Arikan had not wanted his bodyguards with him because they were very young and "he did not want to lose a young boy's life. He said to me, 'Whatever happens to me I don't want to happen to the boys.'