text: With more than half the votes counted, President Juan Manuel Santos is leading his conservative rival Oscar Ivan Zaluaga with slightly more than 50%.
  The campaign has been dominated by discussions on the ongoing peace talks with the leftist Farc rebel group.
  Mr Santos, who launched negotiations in November 2012, lost the first round last month by a narrow margin.
  Mr Zuluaga says he will impose tougher conditions in order to carry on with the talks.
  More than 32 million Colombians were eligible to vote, but the turnout was low in the first round at just over 40%.
  Mr Zuluaga, 55, is a former finance minister. He received 29% of the vote in the first round, compared to 26% for Mr Santos.
  Their main disagreement between the two candidates seems to be on how to end a five-decade long conflict with the country's main rebel group, the Farc.
  Mr Zuluaga initially said he would pull out of the talks and boost military attacks on the weakened rebel group.
  That is the line adopted by his mentor, former president Alvaro Uribe, who was elected senator this year and is still one of the most influential figures in Colombian politics.
  Later in the campaign he said he would continue to negotiate with the Farc provided the rebels renounced military action during the talks.
  Mr Santos, 62, is aiming to conclude negotiations by the end of the year.
  Government negotiators and rebel leaders have held several rounds of talks in the Cuban capital, Havana.
  They have agreed on three points of the agenda drawn up in 2012:  land reform, future political participation and drug trafficking, which is allegedly the main source of income for the rebels.
  Three other points remain to be agreed on: the rights of the victims, disarmament of the rebels and the implementation of the agreement.
  On the eve of the vote, Colombia stopped to watch the national team in its opening match in the football World Cup.
  Both candidates were quick to congratulate the team on its 3-0 victory over Greece in the Brazilian city of Belo Horizonte.
  "This win for Colombia has moved us; and tomorrow [Sunday], we need to wrap things up by winning the presidency," said Mr Zuluaga.
  "Let's go, Colombia. We started things off at the World Cup on the right foot. And tomorrow [Sunday] peace is going to be the winner," said Mr Santos.
  Polls opened at 08:00 local time (13:00 GMT) and closed at 16:00. Results are expected shortly after polls close, as ballots are electronic.
target: Colombians have voted in a runoff election between the incumbent president and a conservative rival.