MEXICO'S old rulers claimed victory in a presidential election on Sunday, ending 12 years in opposition after a campaign dominated by a sputtering economy and rampant drug violence.

After pledging to restore order and ramp up economic growth, Enrique Pena Nieto of the Institutional Revolutionary Party (PRI) had a clear lead over his rivals in exit polls and a "quick count" conducted by electoral authorities.

Although his main rival said it was too early to concede defeat, the 45-year-old Mr Pena Nieto delivered a late-night victory speech to cheering supporters, and a senior electoral official said the PRI candidate's lead was "irreversible".

"Mexicans have given our party another chance. We are going to honour it with results," a visibly moved Mr Pena Nieto told followers packed inside the PRI headquarters in Mexico City.

Jubilant supporters waved banners sporting caricatures of their candidate and his trademark quiff, and confetti in the red, green and white of the Mexican flag - and the PRI's colours - rained down inside the hall.

Outgoing President Felipe Calderon congratulated Mr Pena Nieto on his triumph, which completed a dramatic comeback for the PRI.

With returns in from more two-thirds of polling booths, Mr Pena Nieto had 37% of the vote, more than four percentage points clear of leftist rival Andres Manuel Lopez Obrador. His lead was slowly widening as the night drew on.

Initial projections by Milenio television suggested the PRI had not won enough votes for an absolute majority in either the senate or the lower house of congress.

Mr Pena Nieto's advantage was much less convincing than the PRI had hoped for, with most polls in the immediate run-up to the election showing he would win by 10 to 15 percentage points.

Having run Mexico as a virtual one-party state for most of the 20th century, the PRI was ousted in an election 12 years ago and was seen by many as being on its death bed when it finished way back in third place in the 2006 presidential vote.

Renowned as much for his unfailingly well-groomed appearance as his political skills, the handsome Mr Pena Nieto gave a fresh face to the PRI, helping to instil a new sense of discipline and make the party electable again.

He hopes to use economic reform as a springboard to ignite growth, create jobs and draw the heat out of a drug war that has killed more than 55000 people since late 2006.

He has pledged to boost economic growth to about 6% a year and make bold economic changes, including reforms to allow more private investment in Mexico's state-run oil industry.

The electoral authorities' quick count, an early representative sample from around the country, showed Mr Pena Nieto with about 38% of the vote and Mr Lopez Obrador in second with less than 32%.

Mr Lopez Obrador could choose to challenge the election, as he did six years ago when he narrowly lost to Mr Calderon and launched months of protests against alleged fraud.

He has said in recent weeks that this election campaign was also plagued with irregularities, raising concern that he might again call his supporters onto the streets.

STINGING DEFEAT

Josefina Vazquez Mota of the ruling National Action Party, or PAN, trailed with less than 26% of the vote in Sunday's election.

It was a humiliating defeat for conservative Mr Calderon's party, worn out after a dozen years in power.

Inspiring high hopes when it was elected in 2000, the party has failed to ignite stronger economic growth and Mr Calderon has had no answer to the rampant violence of Mexico's drug war.

"Nothing has improved since the PAN got in," Mexico City plumber Raimundo Salazar said. "The PRI understands how things work here. And it knows how to manage the drug gangs."

The telegenic Mr Pena Nieto built his reputation as governor of the State of Mexico in 2005-11, where he oversaw solid economic growth and brought down the state government's debt.

To his critics, Mr Pena Nieto is a product created by Mexico's main television companies to serve as a proxy for the country's biggest businesses and the ruling elites in the PRI.

"He's been imposed on us by powerful interests like the TV stations and old presidents," biochemist Javier Aguilar said. "How can it be that a country this miserable is home to the world's richest man?" he said, referring to tycoon Carlos Slim.

Mr Pena Nieto's plans include raising tax revenue, a business-friendly overhaul of labour laws and steps to open the struggling state-owned oil giant Pemex to more private investment.

Wary of becoming bogged down in the same battle that exhausted Mr Calderon, Mr Pena Nieto has shied away from targeting the drug cartels, pledging instead to focus on reducing crime.

"The fight against crime will continue, yes, with a new strategy to reduce violence and above all protect the lives of all Mexicans," Mr Pena Nieto said on Sunday night.

However, he dismissed accusations by opponents that the PRI might try to make agreements with drug cartels. "Let it be very clear: there will be no deal, no truce with organised crime."

Reuters