After nearly four years of negotiations, the two sides announce final agreement, which will be put to a referendum.
The Colombian government and the leftist FARC rebel group have reached a historic peace deal to end five decades of fighting that cost hundreds of thousands of lives.
After nearly four years of arduous negotiations in Cuba, the two sides announced on Wednesday a final agreement under which the rebels will lay down weapons and reintegrate into civilian life.
"The Colombian government and the FARC announce that we have reached a final, full and definitive accord ... on ending the conflict and building a stable and enduring peace," the two sides said in a joint statement read out in Havana by Cuban diplomat Rodolfo Benitez.
"We don’t want one more victim in Colombia."
The deal will now be put to a decisive referendum on October 2.
"Colombians: the decision is in your hands. Never before have our citizens had within their reach the key to their future," Santos, who was re-elected in 2014 on the promise of a peace deal, said in a televised address.
"Today I can say - from the bottom of my heart - that I have fulfilled the mandate that you gave me."
The final text of the deal will be sent to Colombia’s Congress on Thursday and will be available on the internet and social media, he said.
"We have won the most beautiful of all battles," lead FARC negotiator Ivan Marquez said after the announcement in Havana on Wednesday.
"The war with arms is over. Now begins the debate of ideas."
The two sides had signed a ceasefire in late June.
The peace deal comprises six agreements reached at each step of the arduous negotiations.
They cover justice for victims of the conflict, land reform, political participation for ex-rebels, fighting drug trafficking, disarmament and the implementation and monitoring of the accord.
Under the peace deal, the FARC will begin moving its estimated 7,000 fighters from their jungle and mountain hideouts into disarmament camps set up by the United Nations, which is helping monitor the ceasefire.
The FARC will then become a political party. Its weapons will be melted down to build three peace monuments.
Special courts will be created to judge crimes committed during the conflict.
An amnesty will be granted for less serious offences. But it will not cover the worst atrocities, such as massacres, torture and rape.
Those responsible for such crimes will face up to 20 years in prison, with lighter sentences if they confess.
Santos insisted there would be no impunity for such crimes.
Most opinion polls suggest Colombians will back the deal but Santos, who has staked his legacy on peace, will face fierce opposition from powerful sectors of the country who believe the only solution is to finish the FARC militarily.
The deal is opposed by two former Colombian presidents, including popular right-wing hardliner Alvaro Uribe.
In Colombia’s capital, Bogota, several hundred people gathered around a giant screen in the rain to listen to the announcement, waving Colombian flags and banners.
"I’m so happy. It was time to end the war," Margarita Nieto, a 28-year-old accountant, told the Reuters news agency. "I know what is coming will be hard, but together we can cope."
Others are more sceptical about the terms of the agreement, especially the participation of FARC rebels in politics and the fact that they will not serve jail time for crimes committed during the war.
"The future worries me," Susana Antequeria, 30, told Reuters. "But I’ll put up with it for peace."
US President Barack Obama spoke by phone with Santos on Wednesday to congratulate him on the deal, the White House said.
More than 220,000 people were killed in the conflict, tens of thousands disappeared and millions fled their homes because of the violence.
The FARC took up arms in 1964 to fight against deep economic and social inequalities and, funded by the cocaine trade and kidnappings for ransom, swelled to as many as 17,000 fighters at the end of the 1990s, controlling large swaths of the country.
But the leftist rebel group were hit hard by Uribe’s government from 2002, when he launched a US-backed offensive that killed many guerrilla leaders and halved their ranks.
An agreement with the FARC does not guarantee an end to political violence. Talks between the smaller, leftist National Liberation Army and the government have stalled.
Key to securing a sustainable peace is additional investment in Colombia’s poorer, rural areas, though deep infrastructure problems across the mountainous nation may stymie progress.