Rebels seemed stunned by the announcement.
“Juan Manuel Santos announced that the cease-fire with the FARC lasts until Oct. 31,” wrote Rodrigo Londoño, the top commander of the rebels. “From then on will the war continue?”
Both sides have said they are committed to preventing the conflict from resuming, and Mr. Santos has the ability to extend the truce. But the range of possibilities to keep the peace now appears dizzying, from quick talks to produce an amended accord to an effort to rewrite the Constitution. Stalled negotiations with Colombia’s second-largest rebel group, the National Liberation Army, look to be thwarted by the public rejection of the FARC deal.
“Oh my God, this has really been so hard,” said María Emma Wills, adviser to the directorate of the Bogotá-based National Center for Historical Memory. “It’s a very sobering moment. It’s a moment of truth.”
On Tuesday, government negotiators were back in Havana, where the talks took place. It was a return to a familiar scene: the two parties sitting at the table, negotiating the fate of the nation.
But the rejection of the peace agreement on Sunday has suddenly changed the political math. Now, the most powerful politician in Colombia is arguably not the president. Instead, it may well be Álvaro Uribe, the senator and former president who led the bitter fight against the accord and prevailed.
Mr. Santos and Mr. Uribe announced on Tuesday that they would meet on Wednesday. Mr. Uribe and his party, the Democratic Center, are preparing an analysis of the flaws they see in the peace deal, laying out their demands for a renegotiated accord.
But at a news conference on Tuesday, María Ángela Holguín, the foreign minister, said that renegotiating the deal “is not a decision of the government, it’s a decision of the FARC,” according to the Colombian news media. The comment infuriated Mr. Uribe’s allies.
“It’s not up to the FARC,” Federico Hoyos, a congressman from Mr. Uribe’s party, said in an interview. “It’s up to the people, and the people decided there needed to be a renegotiation.”
Mr. Uribe and his allies have criticized an array of provisions in the accord. But the most difficult to resolve may be the system of transitional justice it laid out. Under the deal, FARC commanders, even those who had committed crimes against humanity, would have been spared jail time and instead subjected to undefined “restrictions of liberty.”
Mr. Uribe and his supporters have demanded that commanders convicted of the most serious crimes be subject at least to reduced prison sentences, perhaps several years behind bars. Rank-and-file members, they say, could still be given amnesty, as the accord provided.
In addition, Mr. Uribe and his party want to prohibit rebel commanders who have committed heinous crimes from holding elective office, Mr. Hoyos said. The peace accord designated 10 automatic congressional seats for FARC members in the 2018 and 2022 elections.
On Monday, Mr. Uribe also proposed “judicial relief” for the nation’s security forces.
“They know they shouldn’t be the one that really starts the war again, so they’re really trying to move toward peace,” Ms. Wills said. “The big question is what is negotiable between the three big actors.”
The political uncertainty has emboldened Colombians who have long called for an assembly to rewrite the country’s Constitution. Forces on the right, including religious conservatives who back Mr. Uribe, and on the left, including the FARC, have both agitated for years for such a process.
But the idea appears to be highly unpopular among Colombians. Most do not want to overhaul the current Constitution, which was hammered out during a constitutional assembly in 1991, replacing a century-old version and providing a strong defense of minority rights.
“It would probably be a Constitution written by two political extremes,” said César Rodríguez, director of Dejusticia, a legal research institute. “We would achieve peace through a regressive political pact that would change the spirit of Colombian democracy. It’s a big risk.”
While all the main participants in this debate have insisted since Sunday that they want to find a negotiated solution, analysts said that the longer the uncertainty persisted, the greater the possibility that the cease-fire would collapse, plunging the country back into armed conflict.
“If there’s a lack of clarity about where things are headed in Havana, the centrifugal forces are going to be strong,” said Adam Isacson, senior associate for regional security policy at the Washington Office on Latin America, a human rights group.
The FARC is now in a limbo it never encountered during its long insurgency: While the rebels have pledged not to return to the jungles to fight, the referendum leaves them no path out of their camps and into civilian life.
This week was expected to be the start of an exodus of fighters from clandestine camps throughout Colombia to roughly 30 points in the country known as demobilization zones. From there, the fighters — an estimated 7,000 in all — would have had six months to hand over their weapons to United Nations inspectors and begin lives as civilians.
For now, the fighters must wait.
International observers expect the accord’s defeat to delay the release of many child soldiers held by the rebels. In September, the FARC began releasing underage fighters after signing a deal with the government in May.
Philippe Houdard, the founder of the Developing Minds Foundation, a nongovernmental organization that has been involved in rehabilitation programs for child soldiers who escaped, said that many children were likely to remain in rebel hands. While he said that the failure of the referendum would delay their release, a renegotiated deal that appeals to a wider portion of Colombians could benefit them in the long term.
“If Colombia is able to come up with an agreement that’s embraced by a much greater majority of the population, that stigma ex-combatants face will diminish, and the country will say we moved on,” he said.
The rebels may soon have to grapple with a question they believed they had negotiated their way out of: Will members involved in war crimes face tough punishments, rather than the reduced ones proposed under the previous deal?
Colombian negotiators said repeatedly during the talks that the FARC saw jail time as tantamount to surrender and would leave the table before accepting it. But experts say the FARC leaders have little leverage now, given the expectations of the rank-and-file that the war has come to a close.
“Is the FARC going to be willing to accept jail time?” asked Cynthia J. Arnson, the director of the Latin American Program at the Woodrow Wilson International Center for Scholars. “Maybe they will, rather than see this whole thing fall apart.”