After four years of a scandal-weakened government that let the economy slip as guerrillas gained power, Colombians voted for a new president Sunday to succeed the long-embattled Ernesto Samper.

At least six people were killed Sunday in election-related violence. More than 220,000 soldiers and police were on alert to counter an election sabotage campaign by guerrillas, who burned 14 buses, bombed four polling stations, dynamited two electrical towers and abducted seven election officials over the weekend.The front-runners were Andres Pastrana, the Conservative Party candidate whom Samper narrowly defeated in 1994, and the incumbent's friend and handpicked successor, Horacio Serpa of the governing Liberal Party.

Making a late surge in pre-election polls was independent Noemi Sanin, a former foreign minister hoping to break the monopoly on power the two traditional parties have held for more than a century and become Colombia's first female president.

"We've got a very corrupt government, and there's got to be a change," said Bogota lawyer Pedro Guillermo Sanchez, 62, who voted for Pastrana. "We've lived four very negative years, and Serpa represents more of the same."

Cecilia Ripe, a 33-year-old maid, said she was voting for Serpa because "he makes decisions and doesn't waver."

"And he is the one who can bring peace to the country," she said.

None of the candidates is expected to win a majority, in which case the top two finishers will meet in a June 21 runoff. Samper is constitutionally barred from seeking another consecutive term.

Guerrilla threats forced the cancellation of voting in at least two of Colombia's 1,073 municipalities.