With nearly all the votes counted, the candidate of the ruling Liberal party Horacio Serpa won 34.4%, compared to 34.3% for the Conservatives' Andres Pastrana, a former mayor of the capital Bogota and son of a former president.

The run-off will take place on June 21. Attention is already focussing on which of the two will gain most of the votes cast in the first round for the independent, Noemi Sanin, a former foreign minister who had hoped to break the traditional parties' century-old monopoly on power. She won 27% of the vote.

The BBC's Richard Collings in Bogota: 'Peace is the pledge'

At least 11 people were killed and four wounded in election-related violence.

More than 220,000 soldiers and police were on alert to counter a sabotage campaign by left-wing guerrillas.

Fighting between the guerrillas and government troops accounted for eight of the deaths.

A bomb in the strategically-important oil refinery town of Barrancaberneja killed a further three people and injured two soldiers. Police said they suspected bomb was planted by National Liberation Army rebels.

In other attacks in 16 of the country's 32 provinces, rebels are reported to have kidnapped at least 14 election officials. They have also burned ballot papers.

Pastrana: Favourite in the polls

Large parts of southern Colombia are without electricity after two bombs brought down power lines.

Despite the violence, the head of the country's armed forces, General Manuel Jose Bonett, said the public order situation was near normal compared to past elections.

BBC Bogota correspondent Timothy Ross says that the clashes were nothing out of the ordinary in a country where political violence kills 100 people on an average day.

Despite threats by the guerrillas that anyone who venturing out to vote would be shot, a record 11m people turned out to cast a ballot.

In the last election four years ago, two-thirds of the country's 21 million voters abstained.

Violence has flourished

Last year 30,000 Colombians were killed. It is estimated that 3,000 deaths can be directly attributed to clashes between left-wing guerrilla forces, the military and right wing paramilitary groups.

Sanin: Ex-minister but an underdog

That conflict has created an atmosphere in which criminal violence has flourished with the country's notorious drug-traffickers being held responsible for massacres carried out on civilians in rural villages.

Over the last decade about 250,000 people have been killed. Another one million Colombians have been displaced from their homes by violence.

Mr Pastrana says he will be able to mobilise political forces throughout the country and persuade the guerrilla forces to sit down at the negotiating table.

Mr Serpa had been expected to lose votes because he was the outgoing president's interior minister for three years.

President Samper became unpopular with the voters after it was proved in court that his election campaign was financed by the drugs barons.

He is constitutionally barred from seeking re-election when his term ends in August.