Colombians are voting in a presidential election against the background of mounting violence.

Left-wing guerrillas who control a third of the country have threatened to disrupt polling and thousands of extra members of the security forces have been deployed.

Opinion polls suggest a close contest between the three main contenders: the Conservative Party candidate, Andres Pastrana, and Horacio Serpa of the governing Liberals are thought to be narrowly ahead of the independent candidate, Noemi Sanin, who hopes to become Colombia's first woman president.

There'll be a run-off in three weeks' time if no candidate wins more than fifty per cent of the vote.

Under the constitution, the current president, Ernesto Samper, is barred from running for a second four-year term.