Tensions increased Saturday, the eve of the presidential elections, as rebels stepped up their attacks against people violating the transportation stoppage and leftist groups staged protests.

Suspected guerrillas threw a grenade at a police pickup truck in the capital Saturday morning, wounding at least seven people, five seriously, a Red Cross spokesman said. It was the most serious attack in three days of a rebel transport stoppage called by the Farabundo Marti National Liberation Front, or FMLN, to paralyze traffic before the election.The Red Cross spokesman, Alberto Ferreira, said the grenade destroyed the police truck carrying civilians in downtown San Salvador. He said five of the seven victims treated by the Red Cross were hospitalized and two released, but that others were taken to hospital in private cars.

The grenade attack was typical of the FMLN campaign to paralyze the country by intimidating voters into staying home on election day, a campaign that has placed the country under a siege atmosphere but whose success cannot be measured.

The government launched a nationwide effort to persuade voters to go to the polls, turning the military out in massive deployments to protect civilians from attack, manning public buses with army drivers to defy the transport ban and bombarding Salvadorans with radio, television and sound-truck messages.

Most of the country has been without electricity for three days as the FMLN attacked power lines. Explosions at two gas stations earlier in the week persuaded most other stations to close. The few stations that remained open drew long lines of motorists and many residents stocked up on food for the weekend.

In New York, Chicago and Los Angeles, hundreds of people demonstrated to urge an end to U.S. involvement in El Salvador and to support negotiations with the FMLN.

Sunday's elections are to choose a successor to President Jose Napoleon Duarte, the dominant figure in Salvadoran politics for the past eight years.

About 1.87 million Salvadorans are eligible to vote, but turnout is expected to be much lower because of the FMLN threats and the 9-year-old civil war that has left about 70,000 people dead - mostly civilians killed in political violence.

There will be no voting in about 24 of the country's 262 municipalities in areas dominated by FMLN guerrillas, and a number of election officials in other areas have resigned because of guerrilla threats.

Alfredo Cristiani, the candidate of the right-wing Nationalist Republican Alliance, is expected to defeat Fidel Chavez Mena of Duarte's ruling Christian Democratic Party, with Guillermo Ungo of the leftist Democratic Convergence expected to finish a distant third.

But unless a candidate wins more than 50 percent of the vote, the two top vote-winners will hold a runoff election in a month before Duarte steps down June 1.