Former Colombian Defense Minister Juan Manuel Santos has taken a commanding lead in the country's presidential election, but appears to be falling short of the votes needed to avoid a runoff.
With nearly all ballots counted, election officials say Santos has 47 percent of the vote to 22 percent for Mockus.
If Santos' vote total does not rise above 50 percent, he and Mockus will compete in a runoff ballot in June.
Even before Sunday's vote, Santos and Mockus were seen as front-runners to succeed popular President Alvaro Uribe, who has served the maximum two terms. Millions of Colombians went to the polls.
Santos is an economist from one of Colombia's most powerful families. As Mr. Uribe's defense minister, he oversaw the military campaign that largely drove the leftist FARC rebels into the hill and jungle regions.
Santos faced unexpected competition from Mockus, a mathematician and philosopher who campaigned on a message of "clean politics."
President Uribe, who has enjoyed immense popularity for bringing more security to Colombia, has been a strong U.S. ally.
Some information for this report was provided by Reuters, AP and AFP.