Leftist rebels warned that Salvadorans who vote in Sunday's presidential election "will do so at their own risk" and vowed to turn every street into a battleground.

The guerrillas of the Farabundo Marti National Liberation Front crippled transportation in the country Thursday by threatening to ambush public vehicles.Rebels opened fire on a bus in the only reported attack on public transporation since the warning was issued. A 14-year-old girl was killed and a second person was wounded in the ambush, the military reported.

As night fell in the capital, streets were deserted and explosions could be heard in outlying areas of the city.

The rebels vowed to attack polling places throughout the country Sunday when Salvadorans vote in the most critical election in the recent history of the war-torn country.

"Those citizens who try to vote on Sunday will do so at their own risk. Our forces will attack polling places with grenades and machine guns in support of our boycott of these farcical elections," a guerrilla spokesman said on a tape broadcast Thursday on a radio station.

"Every street will become a battleground and we advise people to stay indoors," the statement said."

The FMLN, which has been fighting the U.S.-backed government for nearly nine years, urged supporters to boycott the election after negotiations between the rebels and Salvadoran political parties collapsed. The rebels had proposed the postponement of the elections until Sept. 15 in return for FMLN participation in the campaign.

But Thursday's threats were the first time the FMLN explicitly said it would try to directly disrupt the voting.

Fidel Chavez Mena, presidential candidate of the ruling Christian Democratic Party, told reporters that "the Salvadoran people will not be blackmailed or intimidated. This election will proceed as planned."