A month ago, Rene Tamsen, a reporter covering the political violence in San Salvador for the local radio station WHUR-FM, disappeared. A week later, he was reportedly seen in a Salvodoran prison, his sister says.

Tamsen's relatives, his colleagues at the radio station, friends and others now believe that Tamsen has been imprisoned by the government and security forces of El Salvador.

Yesterday, about 50 of them held a rally in Lafayette Park, across from the White House, to demand that the 28-year-old Tamsen, a native Salvadoan who has been a stringer for Howard University's WHUR radio station for about a year, be freed.

"We consider Rene Tamsen a political prisoner," said Sandra Rattley, news director for WHUR.

Olga T. Aragon, Tamsen's sister who lives in Loudoun County, said she fears her brother will be killed if the United States does not help find him. She read a letter her mother wrote from San Salvador to President Carter, asking for help in locating herson.

A State Department spokeswoman yesterday said inquiries have been made to Salvadoran officials concerning Tamsen, but thus far officials of the Central American nation say they have not been able to locate him. She said the State Department investigation is continuing.

Rattley said Tamsen, who gave his last broadcast from San Salvador on April 17, seven days before his disappearance was here for two weeks in March and April. She said she talked with him about the station's concerns for his safety.

"I think Rene was aware of the possibility that his life was in danger," she said. "He was so committed that he was willing to take that chance."

Tamsen had been reporting on the killings of Salvadoran peasants and children and liberal government officials by some members of the Salvadoran armed forces allied with rightwing extremists. In a live report from Tamsen in January, gunfire could be heard in the background in what Tamsen described as sniping by the military at civilians in the street.

Rattley said Tamsen told her he often teamed with another reporter for safety.

But on the morning he disappeared, Tamsen had left his mother's home in San Salvador alone and was headed to catch a bus to meet a Mexican journalist downtown, Rattley said.

She said witnesses have reported that they saw a man fitting Tamsen's description forced into a taxicab by three civilians in the vicinity of the bus stop.

"He [Tamsen] has always wanted to be a journalist," Tamsen's sister Olga said. "All Salvadorans [here] are concerned about what is going on there. We all worry about it. He never was afraid to go [back to his homeland]. He was El Salvadoran. He felt secure in his work."