Colombian voters rejected a peace deal with FARC rebels Sunday in a surprise outcome that risks prolonging a 52-year-old war and plunges the country into uncertainty.

By a razor-thin margin of 50.21 to 49.78 percent, Colombians voted against the peace accord, in a Brexit-style backlash that defied pollsters’ predictions and left supporters of the deal in tears.

After nearly six years of negotiations, many handshakes and ceremonial signatures, Colombia’s half-century war that has killed 220,000 and displaced 7 million is not over.

“I am the first to recognize the result,” said President Juan Manuel Santos in a televised address, flanked by members of the government peace negotiating team, who looked stunned. “Now we have to decide what path to take so that peace will be possible. . . . I won’t give up.”

Surveys had predicted an easy win for the yes vote by a margin of 2 to 1. Instead, the result was a crushing blow to Santos, who since 2011 has pursued the peace deal with single-minded determination, to the steady detriment of his popularity. He took a significant risk by insisting that the accord — the product of tedious, grinding negotiations with the FARC — would be valid only if Colombian voters gave their blessing.

An opponent to the peace deal signed between the Colombian government and rebels of the Revolutionary Armed Forces of Colombia, FARC, reacts after she listened to the results of the referendum in Bogota, Colombia Oct. 2, 2016. (Ariana Cubillos/AP)

They didn’t, and that failure has left Santos politically crippled. He told Colombians he would send his negotiating team back to Cuba on Monday morning to meet with FARC leaders. Santos also said he would meet with Colombia’s opposition, led by former president and senator Álvaro Uribe, a mortal enemy of the FARC who has gained powerful new leverage over any potential attempt to rewrite the peace deal.

Sunday’s outcome also amounts to a setback for the United States and the Obama administration, which had backed Santos and pledged to boost U.S. aid to Colombia by nearly 50 percent, to $450 million a year. The fate of that funding proposal is also now up in the air.

Bernard Aronson, the U.S. special envoy for the peace process, talked with Colombia’s ambassador in an emergency meeting Sunday night.

“The United States supports Colombia’s democracy and recognizes results of the vote,” Aronson said in an interview, speaking by phone from Washington.

“We believe Colombians want peace, but clearly they are divided about terms of settlement,” he said. “We will continue to support Colombian authorities as they try to build a lasting peace with justice and security.”

The vote was an extraordinary repudiation of the guerrilla commanders of the FARC, who in recent months have tried to engineer a makeover of the rebels’ public image in preparation for an eventual return to politics. The outcome reveals the depths of Colombian public animosity toward the rebels, accumulated by decades of kidnappings, bombing and land seizures in the name of Marxist-Leninist revolution.

Speaking in Havana, where the negotiations have taken place, FARC leader Rodrigo Londoño (alias “Timochenko”) said he “lamented” the results of Sunday’s vote but told Colombians the group remained committed to ending the war.

“We will continue to use words as our only weapons,” he said. “The Colombian people share our dream of peace. Peace will triumph.”

Londoño now faces a major leadership test. Reopening the negotiations will almost certainly mean harsher terms for FARC leaders who have rejected the possibility of prison time. But ordinary FARC soldiers have spent months preparing to lay down their weapons and go home to their families. They will presumably remain in their jungle hideouts, and Santos said a bilateral cease-fire between the rebels and the government will remain in effect.

[A FARC rebel on life in war-torn Colombia]

For many Colombians, Sunday’s referendum was about far more than a cease-fire with the FARC, or Revolutionary Armed Forces of Colombia. Many saw the country’s political and judicial integrity at stake, and they viewed the peace accord as a dubious giveaway to the rebels.

“I want peace, but not if it means kneeling down to the guerrillas,” said Bogota resident Piedad Ramos, 60. “Santos has divided and deceived the country.”

Gina Narvaez, 34, said she voted no because she wants the two sides to “take another look at some of the points of the accord.”

Her brother and her uncle were kidnapped by the FARC in the Huila department in the 1990s. They were freed only after a costly ransom payment.

“They need to change the accord so that there’s some kind of punishment for those who committed these crimes,” she said.

Voter turnout was lower than 40 percent, and heavy rains along Colombia’s Caribbean coast, one of Santos’s strongholds, appear to have sapped support for the accord.

Sunday’s big winner is Uribe, the popular former president who led the campaign against the peace deal. At a base level, Sunday’s contest was a clash between him and Santos, former allies who broke when the president opened peace talks with the guerrillas.

The son of a wealthy Bogota publishing family, Santos is a figure from Colombia’s urban, globalized business elite, for whom the war with the FARC has been a kind of anachronistic developmental constraint. They were hoping that the peace deal would bring a wave of foreign investment and increased trade.

Uribe, whose father, a cattle rancher, was killed by the guerrillas, is beloved by the traditional Colombian landowners who bore the brunt of the FARC’s rural terrorism. And their land disputes with farmers were at the origin of the conflict itself.

In the end, many Colombian voters were skeptical of Santos’s promises of sweeping transformations and appear to have sided with Uribe’s darker vision of the accord as a FARC Trojan horse to take power.

[Colombia’s peacemaker, and his country, on edge ahead of Sunday vote]

Voting got off to a slow start in the capital, where the accord seemed to be doing well.

“I voted yes for the future of my children, so they won’t have to live in a country at war,” said Rocío Cano, 41, a schoolteacher. “Fifty years of violence is enough.”

But others who had suffered personally from the war said they were not ready to forgive the FARC — or at least not through an accord like this one.

“We all want peace, and I respect those who vote yes, but I can’t support this agreement,” said Jakelin Rueda, 33. “There’s no real justice in it.”

Rueda said her father was killed by the FARC in 2002 in the small town of Caparrapi north of the capital, where she grew up. He was a farmer and community leader who opposed the guerrillas.

A lot of city-dwellers voted yes for “idealistic reasons,” Rueda said. “But they have not been affected by the violence directly.”

While FARC leaders did not formally campaign, the rebels made a major last-minute public relations push. For the first time, rebel commanders met with the families of victims at the sites of notorious FARC massacres, seeking forgiveness.

On Saturday, the guerrillas volunteered to get an early start on disarmament, detonating about 1,400 pounds of explosives and other military ordnance in the presence of observers from the United Nations. They apparently didn’t expect to need the weapons again.