(Adds OAS statement, Hernandez rally)
TEGUCIGALPA, Nov 28 (Reuters) - The result of Honduras' presidential election remained in limbo on Tuesday, with a gregarious TV host's surprise lead narrowing sharply, prompting him to call on supporters to take to the streets of the capital to defend the vote.
President Juan Orlando Hernandez, who won U.S. praise for helping tackle the flow of migrants and deporting drug cartel leaders, was favored to win before Sunday's vote in the poor Central American nation with one of the world's highest murder rates.
But a delayed, partial count on Monday morning pointed toward an unexpected victory for TV entertainer Salvador Nasralla, 64. Inexplicably, election authorities then stopped giving results for more than 24 hours.
When, under mounting criticism from international election monitors over a lack of transparency, the electoral tribunal began updating its website again, the tendency rapidly began to change.
In a television interview on Tuesday evening, an angry Nasralla said the election was being stolen from him and asked his supporters to flock to the capital, Tegucigalpa, to protest.
"We've already won the election," he said. "I'm not going to tolerate this, and as there are no reliable institutions in Honduras to defend us, tomorrow the Honduran people need to defend the vote on the streets."
The Electoral Observation Mission of the Organization of American States (EOM/OAS) in Honduras urged people to remain calm and wait for official results, which it said should be delivered as quickly and transparently as possible.
"The credibility of the electoral authorities and the legitimacy of the future president depend on this," it said in a statement.
On Tuesday evening, Nasralla's original five-point lead had thinned to under 2 percentage points, with nearly 71 percent of ballots counted, according to the election tribunal.
Nasralla said in a later television interview that the election tribunal was only counting ballots from regions where Hernandez had won, skewing the results and giving the false sense that the president was heading for victory. He asked the tribunal to include ballots from regions where he was stronger.
A self-described centrist, Nasralla headed a center-left coalition called the Opposition Alliance Against the Dictatorship, and claimed victory on Monday - as did Hernandez.
Election official Marcos Ramiro Lobo told Reuters on Monday afternoon that Nasralla was leading by a margin of five points, with about 70 percent of ballots counted.
Lobo said Nasralla appeared certain to win, signaling that experts at the electoral body regarded his lead as irreversible.
On Tuesday, Hernandez reiterated that he had won, and refused to concede, telling supporters they should wait for final results.
After Hernandez spoke, thousands of his blue-clad supporters gathered outside the presidential residence to celebrate his supposed victory.
"We won the election with Juan Orlando Hernandez, and we won't let them remove him from power," said 35-year-old housewife Maria Aguirre, who hailed from a rough neighborhood on the outskirts of Tegucigalpa.
The election tribunal's delay was due to difficult negotiations between Hernandez's National Party and Nasralla's alliance, according to two European diplomats who spoke on condition of anonymity.
Behind closed doors, the parties were discussing immunity from prosecution for current officials and how to carve up positions in government, the diplomats said.
In an interview on Tuesday, Nasralla denied he was in talks with the National Party. He vowed to review whether to keep a base stationed with U.S. troops if he wins the election, but also promised to deepen security co-operation.
Hernandez's National Party appears set to retain control of Congress in the election, giving it the second-most important perch in the country.
The European Union's chief observer for the election, Marisa Matias, urged election officials to maintain an open channel of communication as they finalized the results.
The electoral body had been so certain Hernandez would win that it showed unprecedented transparency during the contest, one of the diplomats said. That left the body with little room to maneuver when Nasralla came from nowhere to take a strong lead.
With a booming voice and finely coiffed hair, Nasralla is one of the country's best known faces as the host of game shows that feature scantily clad women by his side.
He is backed by former President Manuel Zelaya, who was ousted in 2009 after he proposed a referendum on his re-election. The possible return to a position of influence for one-time leftist Zelaya risks fuelling concern in Washington.
The United States has longstanding military ties to Honduras and few ideological allies among the current crop of Central American presidents.
Hernandez, 49, was credited with lowering the murder rate and boosting the economy, but he was also hurt by accusations of ties to illicit, drug-related financing that he denies.
His bid for a second term, which was made possible by a 2015 Supreme Court decision on term limits, divided opinion in the coffee-exporting nation of 9 million people.
(Editing by Lisa Shumaker and Richard Pullin )