AFP - Colombians have been voting to renew their scandal-tainted Congress in an election that was peaceful but carried the prospect of fraud bankrolled by wealthy narcotraffickers, observers warned.

Polls closed at 4:00 pm (2100 GMT), eight hours after they first opened, allowing 29.8 million voters to pick 102 senators, 166 members and five representatives to Congress.

Despite the continued strength of the Revolutionary Armed Forces of Colombia (FARC) guerilla group, the vote unfolded peacefully.

"No public order incidents affected the electoral process," said General Orlando Paez Baron, a security official, despite an early morning gunfight between troops and FARC guerillas in Cauca that left one soldier dead.

"These are the most peaceful elections we have had on our national territory in the last 25 years," he added.

Observers warned that the main threat to the election's credibility was vote-buying.

"The most sensitive issues... is not violence, but rather vote buying, Organization of American States observer mission head Enrique Correa told AFP.

He said voters in the northern Bolivar region were paid for their votes inside the polling station.

The Colombian Election Observation Mission denounced "the massive vote-buying" witnessed during the poll.

Observers said the fraud was worst in the north-west of the country, where right-wing paramilitaries once held sway.

Observer Pedro Santana said there had been "massive vote-buying, financed by drug trafficking."

Colombians had hoped the vote would give them a chance to oust scandal-tainted politicians, after a slew of charges against legislators.

President Alvaro Uribe's ruling Conservative Party and its allies were expected to keep their majority in both chambers of Congress, despite 12 pro-Uribe legislators being convicted on charges linked to right-wing paramilitary death squads.

The Uribe administration is the top regional US ally in the fight against drug trafficking and attempts to contain the influence of leftist-populist President Hugo Chavez in Venezuela.

But despite being hugely popular for shepherding the country's economy to a strong performance and cracking down on the FARC, Uribe's bid for a third term in office was blocked by a Constitutional Court ruling, and he has no clear successor.

Colombia's Congress is not nearly as popular: in addition to the 12 convicted lawmakers, at least another 80 are under investigation for alleged links to paramilitary groups.

One political party has already been struck from the voting list on suspicion its leaders had links to paramilitary groups, though opposition groups claim the party simply renamed itself and kept the same people, or their relatives, in charge.

Sunday's vote also kicks off internal consultations in the major Conservative and Green parties to pick their candidates for the May 30 presidential election.

Conservatives vying to succeed Uribe include former defense minister Juan Manuel Santos, 58; ex-foreign affairs minister Noemi Sanin, 61; and former senator German Vargas Lleras, 48.

Opposition presidential hopefuls include Rafael Pardo, 47, an economist and former defense minister from the once-powerful Liberal Party, and Gustavo Petro, 50, a former senator, ex-M-19 guerrilla and economist from the leftist Democratic Alternative.

This electoral campaign was the least violent in years, despite the kidnapping and subsequent killing of Caqueta department governor Luis Francisco Cuellar by FARC rebels in December.

Some 210 political kidnappings took place during the 2002 elections, and seven others in 2006.

Police in downtown Cali, Colombia's third most populous city, disarmed a car bomb Sunday set by guerrillas to disrupt voting, and police said rebels set a bus ablaze in the southern province of Tolima.