SAN SALVADOR, JAN. 12 -- Leftist rebels said today they would not allow upcoming elections to take place in areas under their control or in dispute, and warned polticians in those zones against attempting to participate.

"The electoral farce is part of the North American counterinsurgency plan and will be confronted as such," the insurgents said in a broadcast over their clandestine Radio Venceremos, monitored in the capital.

The Marxist-led Farabundo Marti National Liberation Front statement was aimed at March 20 elections for the 60-seat Legislative Assembly and hundreds of municipal officials.

According to diplomats and governmental officials, the elections are an important test of the U.S.-backed effort to establish a functioning democracy in this country that has been torn by civil war for eight years.

The rebels did not say what would happen to those who did not heed their warnings. But, in an apparent bid to show they are serious, the insurgents have kidnaped three politicians in the past week, freeing two of them after warning them not to return to political activity. The third, an official of the Electoral Commission in Cabanas province, is still being held.

"We will not allow any counterinsurgency activity in territory under our control or in dispute," the insurgents said. "We reiterate our call to mayors and government officials not to get involved in these activities, and also issue a warning to businesses that make up or transport materials to be used in this electoral farce."

These will be the first elections since President Jose Napoleon Duarte's Christian Democrats won a slim majority in 1985 legislative elections, capturing 33 of the 60 seats.

El Salvador receives about $1.5 million a day in U.S. economic and military aid, and 55 American military advisers help run the war against the FMLN.

The position of the FMLN could present a problem for its politicial ally, the Democratic Revolutionary Front. Its leaders returned to El Salvador in November, after seven years in exile, to begin open political work. They said they were considering participating in the election, although no decision had been made.

While the FMLN has publicly supported the return of the front's leaders, commanders have made it clear in recent interviews they do not want their allies participating in the electoral process, which they say legitimizes the current rule.

Because stepped-up military sweeps over the past 18 months have forced the rebels to break down into smaller units, the insurgents have less clearly defined areas of control. They are active in eastern Morazan and San Miguel provinces and north-central Chalatenango -- perhaps 25 percent of the country.

Depending on how one defines areas in dispute, the FMLN stance could also disrupt electoral activity in several other provinces, perhaps 50 percent of the country. While the military can provide security, it cannot protect candidates and officials in every town from the threat of guerrilla abduction.

In 1985, the insurgents captured more than two dozen mayors, claiming they were part of the counterinsurgency campaign. Dozens of others left their villages out of fear, and operated out of the provincial capital or other areas deemed safe.

The rebels also abducted Duarte's daughter. The FMLN freed its political prisoners in exchange for government-held political prisoners in 1985. As part of those negotiations, the FMLN agreed to stop kidnaping nonmilitary personnel.