Polls have opened in Nicaragua's presidential election, in which Daniel Ortega, the incumbent president, is expected to win a third term in office, despite controversy over his legitimacy in running for a third term.

Ortega, a one-time far-leftist fighter, who leads the Sandinista party, is riding on a populist platform and World Bank praise for his economic strategies that could help him sweep Sunday's polls.

The supreme court, with a majority of Ortega appointees, set the stage for Ortega to seek re-election when it overruled the two-term limit set by the constitution.

Analysts say a victory for Ortega on Sunday could allow him to make changes to the constitution to end the two-term limit.

With an 18-point lead over his nearest challenger, Ortega is believed to command nearly 50 per cent of voter support.

Al Jazeera's Lucia Newman, reporting from Managua, the capital, said that the elections were off to a controversial start.

She said: "It is a tense situation. There continue to be complaints.

"At the polling place right behind me, opposition members, or people who voted for the opposition, complain that their ballots were made null and void because they did not contain, or bear, the signatures of two members of the voting station as is required by the law.

"They claim that this is because they were voting for the opposition."

While these instances appeared to be isolated, our correspondent also said that it was "much too early to tell how widespread these irregularities are... but the head of the European Union observation team".

Other contenders in Sunday's election include Ortega's closest rival, Fabio Gadea, an opposition politician who runs a radio station.

A third and perennial candidate, and also former president, Arnoldo Aleman, had 11 per cent support in a poll taken between October 10-17 with a margin of error of 2.8 percentage points.

Policies 'broadly favourable'

Since he returned to power in 2007, Ortega has boosted his popularity in Central America's poorest country with a combination of populism and support for the free-market economy he once opposed.

Viewed as a Soviet-backed threat by the US during the Cold War, Ortega led the Sandinista movement that overthrew Anastasio Somoza in 1979, and fended off US government efforts to topple him through a rebel force called the Contras.

Ortega ruled through a junta before he was elected in 1984, but was defeated after one term in 1990. After two more failed runs, he softened his rhetoric, took a free-market stance, and regained the presidency in the 2006 election.

To his supporters, he is just plain Daniel, but critics and opponets say that in his new incarnation, he has espoused "Orteguismo", a politics of personality based on Christianity, socialism and free enterprise.

In his most recent term, Ortega has built wide support among the youth and the poor in a country of 5.8 million people, more than 40 per cent of whom live on less than $2 a day.

He also has maintained ties to the US, even as he has grown closer to Hugo Chavez, the Venezuelan Socialist president.

Ortega has signed the Central American Free Trade Agreement and cultivated Nicaragua's large business sector.

Per capita income has grown steadily since 2006, according to the World Bank, which has praised Ortega's macroeconomic policies as "broadly favourable".

He has been helped immensely by Chavez, who, according to some estimates, has provided at least $500m a year in discounted oil and outright donations to Nicaragua.