MANAGUA, Nicaragua - President Daniel Ortega overwhelmingly won re-election to a third consecutive term in official results announced Monday, putting him in position to govern for a quarter-century and cement family control over the country with his wife now officially vice-president.

With nearly all votes counted, the ticket of the former guerrilla leader and first lady Rosario Murillo captured 72.5 per cent, compared with 15 per cent for the next-closest finisher among five lesser-known challengers. Their ruling Sandinista party is also poised to retain its domination of congress.

But governing during his new five-year term could prove to be a tougher task.

Opposition leaders, who had accused Ortega of rigging Sunday's election and called for a boycott, disputed the official turnout estimate and claimed many people stayed home in protest, undercutting his mandate.

Meanwhile, the Western Hemisphere's second-poorest country faces an uncertain economic future with key benefactor Venezuela in a deepening crisis and other leftist ideological allies losing power in the region.

"I think the biggest problem that Ortega faces is the legitimacy of the process by which he was re-elected, which causes it to be questioned from within and from without the country because it has not complied with rules of fair play," political scientist Humberto Meza said.

"The expectations of the people who are accustomed to subsidies for transportation, subsidies for the cost of energy, social payouts and everything else are going to increase," he added. "I don't think it's an easy thing."

If the 70-year-old Ortega completes his next five-year term, he will have been in power a total of about 25 years, including a decade or so after his Sandinista revolution toppled dictator Anastasio Somoza in 1979.

Critics said the election was unfairly tilted after court rulings that first allowed Ortega to seek a new term and later barred his strongest opponents from running and unseated nearly all opposition lawmakers in congress. They accuse him of creating a new a political dynasty with his wife.

In a statement, the U.S. State Department expressed deep concern over "the flawed presidential and legislative electoral process in Nicaragua, which precluded the possibility of a free and fair election."

Ortega, Murillo and the Sandinistas are genuinely popular in Nicaragua, however, with approval ratings consistently in the upper 60 per cent range thanks to steady if modest economic growth, low violence relative to elsewhere in Central America and social programs that put roofs on homes and pay for school lunches.

"His 40 social projects have helped to partly resolve poverty and basic necessities," former guerrilla commander Eden Pastora said. "These projects will not end all need overnight because Daniel is not a magician, but they have had results ... and that's why they keep re-electing him."

Although still the hemisphere's poorest nation after Haiti, Nicaragua has seen poverty fall in the last decade under Ortega. His supporters will expect him to continue to lower poverty and bolster the social programs.

He will have to do it without the estimated $500 million provided annually by Venezuela from 2007 to 2014 under a program begun by the late Venezuelan President Hugo Chavez. Beset by severe economic contraction and rampant inflation, Venezuela is no longer in a position to continue such largesse.

"I think the most likely challenge that (Ortega's) going to face is economic, particularly as the Venezuelan oil stops flowing," said Gregory Weeks, a political scientist at the University of North Carolina, Charlotte. "Nicaragua has been bolstered so much by Hugo Chavez over the years that it's going to find it rough going."

The U.S. Congress also has pending legislation that would require Washington to oppose loans to Nicaragua from international lending institutions, which could put a further crimp in Ortega's ability to fund anti-poverty measures.

"One of the main challenges is to seek legitimacy for his government, especially from the international community. Ortega knows that now more than ever he must ensure his government's access to the resources of international co-operation and loans from multilateral organizations," Nicaraguan analyst Felix Maradiaga said.

"But," he added, "the excesses of his government have been such that they have made the country a centre of attention and concern for the international community."

Ortega and his critics have long found little to agree on. A day after the election, they didn't even see eye-to-eye on how many Nicaraguans voted.

The Supreme Electoral Council put turnout at about 66 per cent of the country's 3.8 million registered voters. Opponents insisted the number didn't even hit 30 per cent.

"We maintain that what happened was an electoral farce and those results are not in line with the truth," said Violeta Granera of the Broad Front for Democracy. "There was a huge abstention through which the people sent a message to Ortega."

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Associated Press writer Peter Orsi in Mexico City contributed to this report.