Colombians voted Sunday in a crucial presidential election for the fate of the government's peace talks with Marxist guerrillas to end a half-century-old conflict.

The election pitted President Juan Manuel Santos, seeking a second four-year term after launching the peace talks in 2012, against main rival Oscar Zuluaga, who has vowed to take a harder line against rebels.

As he cast his vote in Bogota, Santos said the election "strengthens our democracy, our institutions and regardless of the winner we will continue on the right track towards peace."

Zuluaga, who voted in northern Bogota, said voters were choosing "what kind of country we want."

Amid a field of five candidates, the two rivals were running neck-and-neck in opinion polls leading up to the election, after a late surge by Zuluaga. A likely run-off will be held on June 15.

Voting concluded at 4:00 pm (2100 GMT) and around 90 percent of the results should be known within four hours of polls closing, according to election officials.

Defense Minister Juan Carlos Pinzon said it was the "safest election day in recent history."

Monitors from the Union of South American Nations (UNASUR) said the election was carried out "absolutely normally."

- Allies turned rivals -

Santos and Zuluaga were once cabinet colleagues under former conservative president Alvaro Uribe (2002-2010), but are now bitter rivals and have clashed repeatedly in a campaign marred by espionage and corruption allegations.

Santos, 62, worked as defense minister under Uribe, whose aggressive military campaign led to the killings of key FARC leaders.

But Uribe, who remains popular, has thrown his weight behind Zuluaga, his former finance minister, going as far as calling Santos a traitor for negotiating with the rebels.

The former president reiterated his support for Zuluaga as he cast his vote, saying he chose the best candidate "to retrieve the path of security abandoned by the current government."

Zuluaga, 55, has called for the peace negotiations to be suspended until the rebels give up their weapons.

Santos, in power since 2010, has made ending the conflict the centerpiece of his re-election bid, campaigning on a slogan which offers voters a stark choice: "War or Peace."

The contrasting viewpoints were evident among voters.

"We need a peace accord," said 20-year-old student Maria Paula Erazo.

That way the government could "invest in Colombia's other problems, like health and education," she said.

But one Zuluaga supporter said his candidate would ensure there was "peace without impunity, not what Santos has done.

"His betrayal of the previous government is unforgivable," said Henry Gallan, a 58-year-old seller of security equipment.

The peace process, hosted by Cuba, seeks to end a conflict that has killed more than 220,000 people and displaced more than five million since it erupted in 1964.

"The main issue between the two candidates is the peace process. Santos wants to finish what he started, while Zuluaga's precise intentions are unknown," said Yann Basset, a political scientist at the University of Rosario.

"It's unclear if he's going to terminate negotiations or try to continue them on different terms," Basset added.

The talks have so far led to agreements on rural reform, the participation of former guerrillas in politics and the battle against drug trafficking.

Santos has refused to call a ceasefire during the peace talks in order to keep up pressure on the guerrillas.