By Diego Ore and Marianna Parraga

CARACAS (Reuters) - Supporters of Venezuela's socialist leader Hugo Chavez began setting off fireworks and partying in the street on Sunday night in anticipation of victory over opposition challenger Henrique Capriles in the presidential election.

Various aides to Chavez published celebratory messages on Twitter, while some Capriles supporters began crying at his campaign center, Reuters witnesses said.

"Perfect victory! The fatherland has won," Tweeted Interior Minister Tareck el Aissami.

Various sources on both sides told Reuters that they believed Chavez had won. But there was no confirmation from election authorities who have stressed it is illegal to declare victory before an official announcement.

An energetic campaign by centrist state governor Henrique Capriles, 40, united the opposition and turned him into its best chance in 14 years to unseat the popular president and take the reins of South America's leading oil exporter.

Chavez has used record oil revenue to support ideological allies around the world while preaching a fiercely anti-Washington line, so the election is being watched eagerly from the United States to Belarus and Iran.

"I urge the country to be calm and patient," Chavez said in a phone call to his campaign headquarters.

"Nobody should succumb to provocations, no violence, let's wait for the result."

Earlier, thousands of supporters lined the streets to welcome Chavez as he arrived at the school in a Caracas hillside slum where he cast his vote. Some handed him flowers, and one elderly woman serenaded the president with a folk song.

VIGOR

In a show of vigor, Chavez - who has undergone two operations for cancer since June 2011 - shadow-boxed with U.S. actor Danny Glover, who was on hand with other celebrity fans of the Venezuelan leader to watch him vote.

In poor neighborhoods where Chavez draws his most fervent following, supporters had blown bugles and trumpets in a predawn wake-up call. As dusk fell, hundreds gathered outside the Miraflores presidential palace, partying in anticipation of their man's victory and a speech later from its balcony.

Despite his remarkable comeback from cancer, Chavez, 58, could not match the energy of his previous campaigns - or the pace set by his 40-year-old basketball-loving opponent.

Capriles, a boyish looking lawyer-turned-politician who has never lost an election, rallied support by focusing on the day-to-day problems that worry voters most, such as high crime rates, power blackouts and widespread corruption.

Sporting what he called his "lucky shoes," the superstitious Capriles struck a conciliatory tone after voting. "Whatever the people decide today is sacred," he said to applause from supporters. "To know how to win, you have to know how to lose."

In the United States, Venezuelan expats traveled to New Orleans to vote - mostly for Capriles - after Chavez ordered the country's consulate in Miami closed earlier this year.

There are no formal international observers, though a delegation from the UNASUR group of South American nations, led by an Argentine politician, is "accompanying" the vote.

Local groups were also monitoring the election and both sides say they trust the electronic, fingerprint voting system. The opposition deployed witnesses to all of the 13,810 polling centers, from tiny Amazon villages to tough Caracas slums.

In a politically polarized country where firearms are common and the murder rate is one of the world's highest, tensions have risen in recent weeks. Three Capriles activists were shot and killed by alleged Chavez loyalists on September 29.

After voting, Chavez pledged to respect the election results and called on the opposition to do the same. Some opposition activists fear he could refuse to step down if he loses.

(To follow us on Twitter: @ReutersVzla) (For multimedia coverage, go to http://reut.rs/QzUtvN)

(Additional reporting by Andrew Cawthorne, Deisy Buitrago, Mario Naranjo, Liamar Ramos, Brian Ellsworth, Todd Benson, Daniel Wallis and Girish Gupta; Editing by Kieran Murray, Andrew Cawthorne and Christopher Wilson)

(Note: Venezuelan election law prohibits any publication of results inside the country before an official announcement from the election board)