Colombia resumed its path toward peace with leftist rebels Monday, bolstered by President Juan Manuel Santos' victory in elections seen as a referendum on his bid to end the 50-year-old conflict.

Santos picked up 50.95 percent in Sunday's runoff against the more conservative Oscar Ivan Zuluaga, validating his insistence on continued negotiations with the Revolutionary Armed Forces of Colombia and the smaller National Liberation Army.

Colombians re-elected President Juan Manuel Santos on Sunday in a cliffhanger seen as a referendum on peace talks with FARC guerrillas.

The center-right Santos registered 50.95 percent of the vote, compared with 45 percent for the more conservative Oscar Ivan Zuluaga, according to the electoral board. Another 4.03 percent were blank protest votes.

Colombians went to the polls Sunday in a cliffhanger presidential election that has become a referendum on peace talks with leftist guerrillas.

Polls opened around the country at 8 am (1300 GMT) and were to close at 4 pm (2100 GMT), election officials said.

Colombia on Sunday will hold a presidential run-off that has become a referendum on peace talks under way with Latin America's oldest insurgency.

The mastermind of the talks with the Revolutionary Armed Forces of Colombia (FARC) is the incumbent president, Juan Manuel Santos. His opponent Oscar Ivan Zuluaga is fiercely opposed to the negotiations

Colombia's government and the country's second largest guerrilla group, the National Liberation Army, announced Tuesday they have opened peace talks.

In a joint statement released by the government, the two sides said they have been holding exploratory talks since January "with the objective of agreeing on the agenda and design of the process to make viable the end of the conflict and build the peace."

Negotiators trying to end Latin America's oldest civil war agreed Saturday to set up a truth commission that addresses the deaths of thousands of people in five decades of conflict.

The move is seen as a stride toward a possible peace deal in Colombia.

Thousands of Colombians paid their last respects Wednesday to 28 children who died when an overcrowded bus burst into flames, in one of this nation's worst ever tragedies.

A somber crowd lined the streets as a funeral procession departed the city of Barranquilla en route to the nearby town of Fundacion -- site of tragedy as well as the location of the cemetery where the young victims are to be laid to rest.

The head of Colombia's FARC marked the 50th anniversary of the guerrilla group's founding Tuesday, expressing hope for an "effective peace" while denouncing the government of President Juan Manuel Santos.

In a 30-minute video posted on the Internet, Timoleon Jimenez, alias "Timochenko," warned that the FARC "will do what's necessary if the oligarchy persists in blocking peace."

Colombian President Juan Manuel Santos faces a tough re-election battle after finishing second in first-round elections behind rival Oscar Zuluaga, a fierce critic of peace talks with Marxist rebels.

Zuluaga, a former finance minister, finished first in Sunday's polls with 29.3 percent of the vote compared to 25.7 percent for Santos, forcing a June 15 election because neither got a majority.

Colombian opposition candidate Oscar Zuluaga led President Juan Manuel Santos in first-round elections Sunday, but they now face a runoff in a crucial campaign for peace talks with Marxist rebels.

With nearly all ballots counted, Zuluaga had 29.3 percent of the votes compared to 25.6 percent for his bitter rival, both failing to get the outright majority needed to avoid the June 15 second round.