A group claiming to be the kidnapers of President Jose Napoleon Duarte's daughter is demanding the release of leftist political prisoners in return for her freedom, Salvadoran and diplomatic sources reported today.

The demand, relayed in several telephone calls over the weekend, marked the first word from the abductors of Ines Guadalupe Duarte Duran since she was seized just outside a private Salvadoran university last Tuesday along with a female friend, the sources said. Duarte was allowed to talk with his 35-year-old daughter on the telephone once to confirm that she is still alive, one Salvadoran source close to the government said.

The callers identified themselves as members of the Pedro Pablo Castillo Front, whose formation the rebel movement's Radio Venceremos announced last July 14 as a new war front dedicated to helping the 450 leftist political prisoners held by Duarte's government, the sources said.

A high-ranking military officer said the Pedro Pablo Castillo Front was presented as a new focus of rebel activity, not a new group within the five-member Farabundo Marti National Liberation Front, the umbrella rebel organization that has been waging civil war here for more than five years. The captors of Duarte's daughter probably are members of one of the Farabundo Marti groups acting under the new name to dramatize that they are acting on behalf of guerrilla prisoners, he speculated.

Duarte sent a two-man mission to Mexico City last night to seek contacts with representatives there of the Farabundo Marti National Liberation Front and its political wing, the Democratic Revolutionary Front, according to government and diplomatic sources. The team, Communications and Culture Minister Julio Rey Prendes and Deputy Foreign Minister Ricardo Acevedo, spent the day in the Mexican capital, but there was no word on the outcome of their efforts.

The Salvadoran Roman Catholic Church, the Red Cross and the Mexican government have expressed readiness to act as intermediaries in any contacts, Salvadoran sources said. The church frequently has been a go-between for contacts with the rebel movement and Duarte's government, including negotiations connected with the president's now-suspended dialogue with rebel leaders.

Hector Oqueli of the Democratic Revolutionary Front, or FDR by its Spanish initials, said by telephone from Mexico City that his group had nothing to do with the abduction. FDR officials know nothing about who holds Duarte's daughter and therefore would not be meeting with Duarte's envoys, he added.

Further emphasizing the point, the FDR issued a statement in San Jose, Costa Rica, and Mexico denying any connection with the kidnaping and saying it held no meetings with Rey Prendes and Acevedo.

Oqueli said he did not know if the same holds true for the rebel movement's five military groups. The front's official organ, Radio Venceremos, has maintained silence on the abduction despite wide discussion of it in the Salvadoran press and private radio stations.

Guillermo Ungo in Panama and Ruben Zamora in Nicaragua, the top two FDR leaders, also denied knowledge of the kidnaping in conversations with reporters and said they would not be negotiating with the Duarte government for her release.

Some rebel officials in Mexico expressed fear Duarte's government was turning the spotlight on them in an effort to exert moral and public opinion pressure, forcing FDR leaders to deal with the abduction even though politically it would be more expedient to remain aloof from it.

The FDR, a political and public relations wing of the rebel movement, in the pase has taken stands different from those of the Farabundo Marti National Liberation Front military command and has publicly disagreed with actions carried out by military groups in the front.

Reports differed on the extent of the demands presented this weekend. One source said the callers had a list of nine guerrilla captives to be released in return for Duarte's daughter. But another said the demands included no specific list of names and may have made reference to all 450 political prisoners as a first step in negotiations.

The high-ranking military officer expressed confidence Duarte and the Salvadoran military command would be prepared to exchange prisoners for the woman's release. Such exchanges have taken place on a number of occasions in the past, he pointed out.

The front was named for Pedro Pablo Castillo, a hero of El Salvador's 19th century independence war against Spain.