Former Marxist guerrilla leader Daniel Ortega headed Monday for his strongest ever presidential victory for a third term, according to partial results, in elections marred by irregularities.

The moustachioed leader won 66 percent of the first six percent of votes counted, according to electoral authorities, ahead of right-wing radio host Fabio Gadea, with 26 percent, five hours after polling closed.

"We win," shouted thousands of jubliant supporters of Ortega's Sandinista National Liberation Front (FSLN) in the capital Managua and other major cities, before the announcement of final results.

Ortega, 65, has presided over economic growth in Central America's poorest nation with financial aid from his leftist ally Venezuelan President Hugo Chavez and savvy deals with former foes in the church and business elite.

He faced a fractured opposition in a vote marked by accusations of irregularities and complaints of "difficulties" from international observers.

Dante Caputo, who led the observer team from the Organization of American States (OAS) regional body, said his staff were blocked from 10 of 52 polling stations, out of some 4,200 altogether, mid-way through voting.

The head of a European Union observer team, Luis Yanez, noted "sometimes inexplicable obstacles" and "cheating in a process that should be free."

Tension has risen since a November 2009 Supreme Court ruling cleared the way for Ortega to seek a third term as president. Consecutive re-elections and third terms were supposedly banned.

Several clashes, leaving some two dozen wounded, took place between opposition and Sandinista supporters in Managua and northern areas Sunday, while some of the 3.4 million eligible voters complained of a lack of documents to be able to vote.

In pre-election surveys, Ortega had 48 percent -- 18 points higher than his closest rival, 79-year-old Gadea. To avoid a run-off, he needed more than 40 percent, or at least 35 percent and a lead of more than five percentage points.

Ortega said Sunday he was confident of a sweeping victory.

"This vote will be very high for the Sandinista Front because it is the first time that there is a vote without fear," he said.

Ortega was first elected president of the nation of almost six million in 1984, and has been a central figure since leading a guerrilla movement that ousted dictator Anastasio Somoza in 1979.

US president Ronald Reagan accused the FSLN of fostering revolutionary movements in the region and Ortega was voted out of power in 1990 after a civil war against US-backed Contra rebels.

But Ortega, who had two unsuccessful bids for the presidency in 1996 and 2001, modified his message and crucially reconciled with Cardinal Miguel Obando, a key figure in the staunchly Roman Catholic nation.

Since 2006, Nicaragua's economy has enjoyed steady growth and exports have doubled.

Opponents criticize aid Ortega has received from Chavez -- estimated at more than $1.6 billion since 2007 -- which has propped up popular social programs, including subsidized housing.

Ortega has also attracted foreign investors, who see Nicaragua as a relatively safe haven alongside neighboring Honduras or El Salvador, some of the world's most violent nations.

Nicaragua maintains a free trade deal with the United States, despite Ortega's anti-imperialist discourse and links to Washington enemies such as Iran.

And the former rebel still enjoys solid support in rural and marginalized areas of Nicaragua, where almost half the population still lives in poverty.

Gadea stood on an anti-corruption ticket. Former president Arnoldo Aleman -- sentenced and released on corruption charges -- polled a distant third.

With Ortega so comfortably ahead, his four right-wing rivals focused on Congress, where 90 seats were also up for grabs.