SAN SALVADOR, NOV. 22 -- President Jose Napoleon Duarte today warned leftist political leaders Ruben Zamora and Guillermo Ungo that they can be charged in court in connection with guerrilla "crimes" unless they formally renounce their ties with the country's Marxist-led rebels.

Duarte issued the warning a day after Zamora, the vice president of the Democratic Revolutionary Front (FDR from its Spanish name), a leftist political grouping, returned from seven years of exile to resume political work inside the country under the terms of a Central American peace accord. Ungo, the president of the FDR, is scheduled to return Monday in the company of several followers and a group of international observers.

In view of the exiles' return, the Farabundo Marti National Liberation Front (known as FMLN), an organization of five guerrilla groups that is allied politically with the unarmed FDR, announced a unilateral nationwide cease-fire effective today and Monday. In a clandestine radio broadcast today, the FMLN said that afterward it would observe a "partial truce," in the capital and the surrounding province, from Wednesday to Monday.

Duarte also announced today a development that he termed a breakthrough toward solving the 1980 assassination of Archbishop Oscar Romero.

In a press conference after a speech to a convention of his Christian Democratic Party, Duarte said that if Zamora and Ungo fail to break with the country's rebel movement, "any crimes committed by the FMLN during these days are their responsibility." He added, "If Mr. Zamora says he continues to be connected to the FMLN, any citizen of the country can go to any court and present a case for all the crimes committed by the rebels."

The warning, which amounted to an invitation to the public to file charges against the returning exile leaders, appeared to be aimed at heading off criticism of the Duarte government from the country's powerful right wing. Duarte expressed concern that if the leftist politicians did not renounce the FMLN, "the extreme right is going to take advantage and say this is a weak government."

Regarding the case of Romero, the Catholic archbishop who was assassinated by a suspected rightist death squad in 1980, Duarte said the driver of the getaway car used by the killers gave testimony to investigators yesterday during a one-day visit from another country where he now lives.

"It's the first time that anybody has given any real testimony in the case," a diplomat said.

Duarte refused to comment further, saying he would release more details Monday. He denied that the timing of the announcement was politically motivated, arguing that investigating the Romero case "is a long process."

A U.S. Embassy official declined comment when asked where the driver lives abroad and whether the United States had played a role in obtaining his testimony.

Duarte's announcement came shortly after Zamora attended a mass at San Salvador's bullet-pocked, earthquake-damaged cathedral and laid a wreath on Romero's tomb in a side chapel. Wearing a dark suit and glasses and a bullet-proof vest, Zamora then prayed at the tomb under a large portrait of the assassinated archbishop as members of the congregation applauded.

Sitting between an American priest and a U.S. congressional staff member who accompanied him on his flight here from Mexico yesterday, Zamora -- an ex-member of Duarte's party -- listened as the archbishop of San Salvador, Arturo Rivera y Damas, welcomed his return and criticized recent actions by both the guerrillas and the government.

Rivera y Damas said he hoped Zamora's presence "signifies a contribution to dialogue and, eventually, to peace." He charged that rebels in San Miguel province recently forced church workers to perform certain "tasks" that placed their lives in danger. And he criticized the "violent arrests" of a family of bakers and their employes by the Army recently on charges of belonging to an FMLN cell on the outskirts of the capital.

In his news conference, Duarte suggested that if any harm comes to Zamora or Ungo, their guerrilla allies should be held responsible.

"My greatest worry is that something might happen, and something can happen because the guerrillas can attack them," Duarte said. "For the guerrillas, that would be the maximum benefit they could derive. I worry that the guerrillas have invited them inside {the country} in order to sacrifice them."

At another point, Duarte said that "Zamora has come back because things have changed, and this destroys the thesis of the FMLN and the FDR that things have not changed." But he said he suspected that the two fronts might be following a dual strategy of waging "protracted people's revolutionary warfare" and, at the same time, "inserting themselves into the democratic process in order to destroy the democratic process."

Zamora left El Salvador shortly after his older brother Mario, a public defender under the country's former ruling junta, was killed by a death squad in his home during a party in February 1980. Later that year, five leaders of the FDR were gunned down by death squads here.