GUATEMALA CITY (Reuters) - Still reeling from a corruption scandal that felled their president three days ago, Guatemalans voted on Sunday to elect a new leader in a tight contest that is likely to head to a second-round run-off.

Otto Perez resigned as president on Thursday and was jailed while a judge weighs charging him over a customs racket. The affair has gutted his government and plunged the poor Central American country into its worst political crisis in decades.

Voter anger over corruption has helped a little-known comedian to surge in opinion polls, while the three main contenders have vowed a crackdown on graft after mass protests on the streets.

Polls in the run-up to Sunday's vote showed Jimmy Morales, a 46-year-old centrist and comic actor whose slogan "not corrupt, not a thief," has resonated with disenchanted voters, going head-to-head with earlier favorite Manuel Baldizon, 45, a conservative businessman.

"We have suffered such great disappointments that you end up telling yourself there is no one to put your trust in," 38-year-old housewife Lidia Mendoza said shortly before polling booths closed. She voted for Morales precisely because he was an outsider.

After maintaining a sizeable lead over Morales for months, Baldizon had around 23 percent support, just shy of Morales' 25 percent, heading into the vote.

While Morales has not laid out a clear political agenda, he has vowed to fight poverty by improving the education system and decentralizing the budget and government powers.

"Guatemala wants change and to not be governed by people with dark pasts," said a smiling Morales after voting near Guatemala City, flashing a victory sign to reporters. "I planted love in my homeland and now I'm harvesting that love."

Baldizon, a congressman for the center-right opposition Renewed Democratic Liberty Party (Lider), promises to combat tax evasion, promote government austerity and modernize the state, while also pledging to curb graft.

"Guatemala deserves that we make an effort to raise up a country destroyed by corruption," Baldizon, a fervently religious lawyer, told a political rally on Saturday.

But corruption allegations have also smudged the ticket, with Lider's vice presidential hopeful Edgar Barquin, a former central bank chief, accused of criminal association and influence trafficking by a powerful United Nations-backed anti-graft commission. He has not been charged.

"Who did I vote for? Nobody, because they're all thieves," said 58-year-old engineer David Garcia as he exited a Guatemala City school after casting a blank vote. "I'd rather invite them into my house to steal everything from me than vote for them."

Leftist candidate Sandra Torres, the ex-wife of former President Alvaro Colom, narrowly trails Baldizon and Morales. She has vowed to fight poverty by increasing social spending by 0.5 percent of gross domestic product.

Mario Garcia, the candidate from Perez's right-wing Patriot's Party, is polling well behind the front-runners.

If, as expected, no candidate wins more than 50 percent of the ballots cast by Guatemala's 7.5 million registered voters, the top two will face a run-off on Oct. 25.

Perez, a retired general who came to power in 2012 promising to be tough on crime, was set to leave office in January. Following his resignation, Congress transferred power to his vice president, Alejandro Maldonado.

As leader of Central America's largest economy, Maldonado's successor will be tasked with tackling a stubbornly high poverty rate, despite nearly uninterrupted economic growth since the end of a 1960-96 civil war.