SAN SALVADOR, FEB. 19 -- Leftist rebels, saying they were headed toward "definitive battles" with the military, called their first national transportation stoppage of the year today and stepped up attacks across the country to protest next month's elections.

Also this morning, unknown assailants set off three bombs in different parts of the capital, damaging buildings but causing no injuries.

In a broadcast over their clandestine Radio Venceremos, the guerrillas said they had issued orders to all their units to destroy any vehicle found on roads after midnight Sunday, and said the strike would last indefinitely.

They also warned bus drivers in urban areas not to drive, saying the vehicles would be destroyed.

The Farabundo Marti National Liberation Front, made up of five Marxist-led armies, has promised repeatedly in recent months to launch a major military offensive, and vowed to disrupt municipal and legislative elections, scheduled for March 20.

Col. Mauricio Ernesto Vargas, head of military operations, said the military would deploy thousands of troops along the nation's main highways in an effort to guarantee travelers' safety, but, if past experience is a guide, few will venture out, at least during the first days.

The rebels and the government today reported heavy fighting near the town of La Laguna, in north-central Chalatenango province, and near Suchitoto, in Cuscatlan province.

Reuter reported today that two California church workers, Mary Parmenter of Los Angeles and Kathy Hoy of Pasadena, said the government removed them and seven Salvadoran colleagues yesterday from a resettled village in a war zone in Cuscatlan province.

They said they were held for most of yesterday, prevented from making a phone call for several hours, interrogated by treasury police in the capital, then released unharmed after a U.S. Embassy official went to police headquarters.

A military spokesman said the women did not have permission to be in a war zone, but he declined further comment.