Colombians were voting in a referendum on Sunday that will either affirm a historic peace accord - or destroy it.
The nationwide vote asks Colombians if they support the peace accord signed last week between the government and the Revolutionary Armed Forces of Colombia (FARC) guerrilla group after 52 years of conflict and nearly four years of talks.
President Juan Manuel Santos took the unusual step of making final approval by Colombia's people a condition of the peace deal, and has assembled a broad coalition of supporters urging a "yes" vote.
But opponents of the accord, led by former president and current far-right Senator Alvaro Uribe, say it concedes too much to FARC and are campaigning for "no."
Opinion polls suggest Colombians will approve the peace accord, showing between 55 and 66 per cent of likely voters supporting the deal.
The winning side must reach a threshold of at least 13 per cent of the 35 million eligible voters - 4.5 million people - for the vote to be declared valid.
If the deal is approved, it will take effect immediately. Peacetime will begin with a six-month process to demobilise 6,000 FARC guerrillas, who will turn over their weapons to UN inspectors.
If it is rejected, the final accord is effectively scrapped, and with it the plans for peace. Both sides have said they are unwilling to renegotiate the terms, and while some analysts say a restart to talks is technically possible, others say a return to hostilities is too.
The armed conflict between Colombia and FARC began in the 1960s over inequality and rural land rights. In more than a half-century since then, the violence has killed more than 220,000 people and displaced millions.