Colombia’s President Juan Manuel Santos, left, and the top commander of the Revolutionary Armed Forces of Colombia (FARC) Rodrigo Londono, known by the alias Timochenko, shake hands after signing a peace agreement. (Fernando Vergara/AP)

Colombian President Juan Manuel Santos and the leader of the country's main guerrilla group signed a historic peace accord Monday, in a ceremony meant to promote their agreement to end the country’s 52-year armed conflict.

Held in the colonial city of Cartagena, the event marked the first time that Santos and Rodrigo Londoño, leader of the Revolutionary Armed Forces of Colombia, or FARC, publicly appeared together on Colombian soil.

U.S. Secretary of State John F. Kerry, U.N. Secretary General Ban Ki-moon, Cuban President Raúl Castro and other heads of state attended the ceremony, all of them dressed in white, as a show of support for the pact.

As a binding document, though, the agreement will matter little unless Colombian voters approve it in a crucial referendum on Sunday.

By scheduling the signing ceremony ahead of the vote, the Santos government and the guerrillas hope to boost public support for their accord and send the message that the pact is irreversible. Both sides say the agreement — the result of four years of painstaking negotiations — is a finished product whose rejection by voters would simply prolong a conflict that has killed 220,000 people over the past half-century.

To chants of “no more war!” from an audience dress in white, symbolizing peace, Santos told Colombians in an emotional speech that their long “horrible night of violence is over,” and that “a new dawn of peace is here.”

In a statement aimed at critics of the deal, Santos said “every pact is imperfect,” but called the accord with FARC “the best deal possible.”

“I would rather have an imperfect agreement that saves lives than a perfect war that continues sowing death,” he said.

He was preceded by FARC leader Londoño, who drew the biggest applause when he apologized on behalf of the guerrillas to “all the victims” of the conflict “for any pain we have caused.” Colombian military jets streaked overhead as he finished his speech, startling him but bringing a smile.

Londoño said the rebels were ready to fully embrace Colombian democratic politics. “Our only weapons will be our words,” he said.

The latest surveys indicate that the accord will be approved by a wide margin on Sunday, but many analysts say the outcome could be closer than expected. Former president and Santos archrival Álvaro Uribe has led a campaign against the deal with the slogan Paz Sí, pero No Así (Peace Yes, but Not Like This), calling the accord a disgraceful and costly capitulation to a terrorist group.

Santos pursued the accord with a single-minded determination that will probably make him a leading candidate for a Nobel Peace Prize if voters back the deal. He also insisted on giving Colombians the final say, submitting the agreement to a democratic vote.

But the unconventional tactic could backfire if voters don’t go along. With the country’s economy slowing, Santos’s approval ratings are slumping, and public perceptions of his peace partner — the FARC — are even more negative.

Londoño, better known by his guerrilla alias Timochenko, remains an unfamiliar sight to a Colombian public that until recently saw him mostly on “Wanted” posters. He and other FARC commanders led a gathering of rebel forces last week in a remote part of eastern Colombia under guerrilla control, but he has never appeared at a public event in a major city.

The accord calls for the FARC’s 7,000 or so fighters to move into camps — which would be monitored by the United Nations — to begin handing over their weapons. In one of its more controversial provisions, it would guarantee the rebels 10 seats in Colombia’s congress for two terms, opening a path for rebel commanders to enter politics.

The leftist guerrillas have not announced a name for their future political party.

Kerry told reporters on Monday that the FARC’s presence on the U.S. list of terrorist organizations remains unchanged but could be reviewed once the peace accord is implemented.