Tension has grown over the outcome of presidential elections in Honduras, where an early victory declaration brought angry protests.
Mr Zelaya promised a new era of transparency and justice
The country's senior electoral official declared opposition candidate Manuel Zelaya the winner, even though only 1% of votes had been counted.
His opponent, Porfirio Lobo, decried the announcement, demanding that every ballot be tallied.
Outgoing President Ricardo Maduro has asked both candidates for restraint.
"We have a president-elect," Aristides Mejia, the head of the country's electoral tribunal, told Reuters on Monday.
"I believe the difference is clear and conclusive."
Mr Lobo, from the ruling party, has refused to admit defeat
Mr Zelaya promptly claimed victory, and his supporters broke into celebrations, with singing and dancing at his Liberal Party's campaign headquarters.
However results giving Mr Zelaya 50% of the vote to Mr Lobo's 45% were only gathered from a small number of polling stations.
Other officials cautioned that the full results may take days or weeks to come in.
Mr Lobo refused to accept defeat. "I'm going to defend the right of my people to have their votes counted," he said.
Sunday's vote was the seventh presidential election since 1981, when civilian rule was restored.
Both right-wing leading candidates, who were neck-and-neck going into the polls, vowed to cut crime in Honduras, which is wracked by gang violence, 30% unemployment and poverty.
Mr Lobo, 57, is a former communist who has pledged to introduce the death penalty for crimes such as sexual assault, kidnapping and murder.
Mr Zelaya, 53, is a civil engineer and rancher who has previously served as investment minister, and wants to maintain life sentences for the worst offences while doubling police on the streets.
The three other candidates who contested the election were not expected to draw widespread support.
Mr Lobo worked alongside current President Ricardo Maduro, a National Party colleague, to introduce penal code reform that has criminalised gang membership.
But there has been little let-up in the violence, with at least 45,000 gang members estimated to be operating inside Honduras.
The gangs grew out of Los Angeles and have become a fixture of life in Honduras. The most notorious group, the Mara Salvatrucha, is blamed for a bus massacre that killed 28 people in late 2004.
An estimated four million registered Honduran voters have also elected 298 mayors and 128 deputies to the single-chamber Congress.