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 $US390 million ($510 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.