Juan Manuel Santos was re-elected over the weekend to a second term as Colombia's president, winning with the support of the left and grassroots organizations that backed his agenda of continuing the peace process with guerrillas and achieving social justice.

Santos received 50.95 percent of the vote in Sunday's runoff election, while challenger Oscar Ivan Zuluaga got 45 percent of the ballots cast, the national elections commission said.

"To those who voted for the simple reason of supporting peace and are critical of our policies on other fronts, I tell you that I will take on the great challenges so that in four years no one will wish they had not voted for us, we will not fail you," Santos said in a an address following the release of the election results.

The 62-year-old Santos, who ran as the candidate of the National Unity coalition, made reaching a peace agreement to end the conflict with Colombia's guerrilla groups the focus of his campaign.

Santos forged an alliance with leftist movements following the May 25 first-round presidential election, which Zuluaga, of the Democratic Center party, won with 29.25 percent of the vote to the president's 25.69 percent.

"Many thanks to all the leaders of the left, the progressives, the independents, the intellectuals, the artists, the grassroots, women's, victims', peasants', Afro-Colombian, Indian and environmental movements, to the teachers, to the unions that joined us because they share our desire for peace with a social focus," Santos said.

Santos said his victory made possible "the end of more than 50 years of violence and the start of a new Colombia with more liberty, with more social justice."

The 55-year-old Zuluaga, who was backed by former President Alvaro Uribe, has been a harsh critic of the Santos administration's peace negotiations with the Revolutionary Armed Forces of Colombia, or FARC, guerrilla group.

The FARC declared a cease-fire from June 9 to June 30 and the smaller National Liberation Army, or ELN, rebel group said it would respect the election process but would not declare a cease-fire.

Santos's second term will run from 2014 to 2018.