Quito: Ecuador plunged into crisis on Sunday night after a disputed presidential vote, with leftist candidate Lenin Moreno headed to a narrow victory and his conservative opponent denouncing the results as fraudulent.

With nearly 95 per cent of the ballots counted in the second-round presidential run-off, Moreno led by 51 per cent to 49 per cent over right-wing challenger Guillermo Lasso, who insisted he was the real winner.

Moreno was the candidate of the ruling PAIS alliance and the hand-picked successor to outgoing President Rafael Correa.

Clashes broke out in several cities, with many fearing an escalating stand-off and Ecuadoran voters screaming at one another in the streets.

Citing an exit poll by the respected Cedatos firm showing him winning by a comfortable margin, Lasso gave an emotional speech declaring victory as soon as polls closed.

"Fight!" he told his supporters, well before the first official tallies were released.

"We won't let them cheat us!"

At a rally soon after, Moreno told his cheering supporters that he had won.

"Onward to victory!" he shouted. "We'll continue changing Ecuador for the better."

On Twitter, Correa said violence had broken out in Quito and several other cities.

"What they can't accomplish at the polls, they're trying to achieve by force," he wrote, after declaring Moreno, his former vice-president, the winner.

"The revolution has triumphed again in Ecuador," he said.

But the government's opponents seemed in no mood to accept that result, demanding a recount.

"We demand the truth," said opposition leader Cesar Monge, holding up copies of vote tally sheets that he said were proof of tampering. "We reject this."

Also at stake in the outcome was Julian Assange's asylum protection at Ecuador's embassy in London, because Lasso has said that he would evict Assange within 30 days from the embassy, where the WikiLeaks founder took refuge in 2012.

Moreno has said he would let Assange stay.

Lasso and his supporters began celebrating in the streets of the capital as soon as exit polls showed him winning - waving flags and honking car horns wildly.

After rallying outside the headquarters of the country's election authorities, they broke through police barricades and surged towards the building, with television cameras showing them facing off against riot police with shields.

The disputed outcome is one of several South American conflicts that have occurred in recent days, along with clashes in Venezuela and Paraguay.

Election observers from the Organisation of American States and other organisations had yet to make statements about the integrity of the vote.

A respected non-governmental organisation, Participacion Ciudadana, said its exit poll results showed a tie between the two candidates.

Correa's decade in power has left Ecuadorans sharply divided and, with his legacy on the line, his government has thrown its full weight behind Moreno, 64.

Lasso, 61, a former banker, offered Ecuadorans a message of "change", and bet that frustration about the country's sagging economy and Correa's heavy-handed style would lift him to an upset.

"We need new ideas. Everything is stagnant here," said Luzmila Munoz, 47, a chemical engineer who voted for Lasso in a middle-class sector of Quito.

"Ten years is enough," she said, referring to Correa, who is ineligible for re-election.

Right-wing candidates have won recent presidential contests in Argentina and Peru, after a long period of dominance by left-wing populists such as Correa, who used a commodity boom to cut poverty and cultivate a broad base of support.

But with prices for oil and other exports slumping, the region has shifted to the right, and many leftists saw the mild-mannered Moreno as their best chance to break the trend. Moreno, who was shot in a 1998 carjacking, would be the first candidate in a wheelchair to win a presidential race in Latin America.

"He'll fight for equality, because he knows what it's like to be disadvantaged," said Janet Bravo, 40, who cast her vote for Moreno in the rough hillside neighbourhood of Comite del Pueblo. Bravo, who owns a small office supply shop, said she has been able to save money in recent years because the government provided her two small children with free healthcare. "I want government to continue along this path," she said.

Moreno's campaign was counting on voters such as Bravo to be wary of what sort of change a Lasso win would bring to their lives.

"I'm afraid we'll go back to the way things were before," said Erick Lara, 22, an Afro-Ecuadoran who is studying to be a chef. He credited the Correa government for promoting racial equality and said his mother was able to buy her own home thanks to a government loan.

"We have more opportunities now," he said.

Michael Shifter, president of Inter-American Dialogue, a Washington think tank, said Moreno's apparent victory showed that left-wing governments in South America might be more resilient than many believe.

"Although Ecuador's economic situation has recently worsened and there are serious questions about government corruption, most voters recognised advances in education, healthcare and especially infrastructure," he said.

"Moreno promised to give a new push and build on these gains."

But the disputed, narrow results suggested Moreno will face immediate challenges in governing a badly divided country in an increasingly volatile region.

The Washington Post