Colombians hit their government with a shock defeat Sunday when they voted by a razor-thin majority to reject a historic peace accord with communist FARC rebels.

Voters resentful of the blood shed by the FARC narrowly defied the government's bid to put the 52-year conflict behind them, reversing the trend of earlier opinion polls.

The result threw Colombia's future into uncertainty. The sides spent four years negotiating the deal and agreed it must be ratified in a referendum -- but said there was no Plan B.

The 'No' camp won by about 54,000 votes which translated into a lead of less than half a percentage point, electoral authorities said.

President Juan Manuel Santos admitted defeat in the vote but vowed: "I will not give in, and I will continue to seek peace to the last day of my term."

FARC chief Rodrigo Londono, alias Timoleon "Timochenko" Jimenez, vowed his side too was committed to continuing peace efforts. He said its ceasefire remained in force.

"The FARC deeply deplores that the destructive power of those who sow hatred and resentment has influenced the Colombian people's opinion," he said in a speech in Havana, Cuba, where the accord was negotiated.

"The people of Colombia who dream of peace can count on us. Peace will triumph."

- Deadly conflict -

Supporters of the accord had expected it to effectively end what is seen as the last major armed conflict in the Western hemisphere.

But Sunday evening's result was a dramatic defeat for Santos and the accord he signed with the Revolutionary Armed Forces of Colombia (FARC).

Commentators compared the drama of the result to that of June's surprise "Brexit" vote for Britain to leave the European Union.

Colombians voted 50.21 percent to 49.78 percent against the accord, according to results published online with more than 99.9 percent of votes counted. Turnout was low at just over 37 percent.

Authorities earlier said heavy rain caused some disruption to voting as Hurricane Matthew passed over the Caribbean.

- Hatred of the FARC -

Some victims of the FARC had said publicly that they backed the accord.

But forecasts apparently miscalculated Colombians' desire to punish the FARC.

Opponents of the deal resented the concessions offered to the armed group.

They included an amnesty for some FARC members, though not for the worst crimes such as massacres, torture and rape.

"It is absurd to reward those criminals, drug traffickers and killers who have made the country a disaster for the past 50 years," said No voter Jose Gomez, a retiree of 70.

Monica Gonzalez, 36, celebrated the result in northern Bogota on Sunday night. She said the FARC killed her grandmother in 2011 and kidnapped some of her other relatives.

"I agree with second chances, but not with impunity," she said.

- 'Political crisis' -

The leader of the 'No' campaign, former president Alvaro Uribe, called for a "national pact" to work for peace. But it was unclear how peace efforts might move forward now.

"Hatred of the FARC won the vote," said Jorge Restrepo, director of conflict analysis center CERAC.

"We have been cast into a deep political crisis with very negative economic consequences."

Supporters of the 'Yes' vote who had prepared to celebrate on Sunday night at a hotel in central Bogota were left in gloom.

"No one was prepared for this. There was no Plan B," said one Yes supporter, Jorge Cifuentes, 55.

"We do not know what will happen now, but it is clear that the conditions granted to the FARC had a big effect, and the low turnout too."

- Deadly conflict -

The accord called for the 5,765 FARC rebels to disarm and convert into a political group with seats in Colombia's Congress.

The accord covered justice and compensation for victims and an end to the cocaine production that has fueled the conflict.

The FARC launched its guerrilla war on the government in 1964, after a peasant uprising that was crushed by the army.

The ideological and territorial conflict drew in several leftist rebel groups, right-wing paramilitaries and drug gangs.

Colombian authorities estimate the conflict has left 260,000 people dead, 45,000 missing and nearly seven million displaced.