Colombian President Juan Manuel Santos and Marxist rebel leader Timochenko have used a pen made from a bullet to sign an accord ending a half-century war that killed a quarter of a million people.
After four years of peace talks in Cuba, Santos, 65, and Timochenko - the nom de guerre for 57-year-old revolutionary Rodrigo Londono - shook hands on Colombian soil for the first time in front of hundreds of dignitaries.
One man waved a large Colombian flag that had an extra white stripe in homage to the peace deal.
The end of Latin America's longest-running war will turn the FARC guerrillas into a political party fighting at the ballot box instead of the battlefield they have occupied since 1964.
Guests at the ceremony in the Caribbean coastal city of Cartagena were asked to wear white and included United Nations Secretary-General Ban Ki-moon, Cuban President Raul Castro and US Secretary of State John Kerry.
Showing its support for the peace deal, the European Union on Monday removed the FARC from its list of terror groups.
Kerry said Washington will also review whether to take the FARC off its terror list, and has pledged US$390 million for Colombia next year to support the peace process.
"Anybody can pick up a gun, blow things up, hurt other people, but it doesn't take you anywhere... Peace is hard work," he said of a rare diplomatic good news story for the Obama administration as it contends with the seeming intractable war in Syria and other conflicts.
Despite widespread relief at an end to the bloodshed and kidnappings of the past 52 years, the deal has caused divisions within Latin America's fourth-largest economy.
Influential former President Alvaro Uribe and others are angry the accord allows rebels to enter parliament without serving any jail time.
Colombians will vote on October 2 on whether to ratify the agreement, but polls show it should pass easily.
In Cartagena on Monday, huge billboards urged a "yes" vote, while Uribe led hundreds of supporters with umbrellas in the colors of the Colombian flag urging voters to back "no".
The FARC, or Revolutionary Armed Forces of Colombia, began as a peasant revolt, became a big player in the cocaine trade and at its strongest had 20,000 fighters. Now its some 7000 fighters must hand over their weapons to the United Nations within 180 days.