All seven opposition leaders who contested Guinea's presidential election against incumbent Alpha Conde said on Monday the result should be annulled because of fraud.

Their declaration is likely to stoke tension in the West African country, which has a history of political violence, including at the 2010 election that brought Conde to power.

Conde, who rose to power in a military coup, is favored to win a second term, although the result from Sunday's vote may be close enough to require a second round. Early results announced by radio stations so far showed Conde in the lead.

The opposition candidates, including the main opposition leader, Cellou Dalein Diallo, told a news conference that there were numerous examples of fraud in the election.

Diallo said voters registered this year in the city of Labe in central Guinea received no voting cards and only those who voted in 2010 could cast their ballots on Sunday.

"The election was a masquerade which started yesterday and still continues today at the central (election) commission level. In these conditions, we again demand that the election be scrapped because we cannot recognize results issued through this process," Diallo said.

"We have the right to protest. We will do it. That must be clearly understood," he said, in a declaration that appeared to stop short of calling his supporters into the streets.

Two people were killed and at least 33 were wounded in Guinea on Friday in clashes between Conde and Diallo's supporters.

In another sign of opposition discontent, former prime minister Sidya Toure, one of the election's leading candidates, told private radio late on Monday that he was withdrawing any delegates he had won in the vote from the electoral process.

U.N. Secretary General Ban Ki-moon appealed in a statement for calm and urged all sides to refrain from making statements that could lead to violence.

Conde's director of communication, Moustapha Naite, said the electoral commission should do its work.

"If the opposition parties have complaints, their duty is to legal means and inform the constitutional court," Naite said, referring to the country's highest electoral authority.

The International Organization of the Francophonie, whose experts have been assisting the electoral commission since March, praised Sunday's vote but urged caution.

"What we saw yesterday was impressive. People turned out en masse in calm and serenity," Francophonie chief of mission Mohamed Salia Sokona told Reuters.

"It is one thing to go to the polls. Results are something else. We hope that this conduct prevails and that Guineans may show a sense of responsibility and democracy," he said.

Military officers seized power in a 2008 coup following the death of longtime ruler Lansana Conte. The next year, security forces killed more than 150 pro-democracy protesters who had rallied at a stadium in the capital and raped dozens of women.

Conde was elected in 2010 in Guinea's first democratic handover of power since independence from France in 1958.

(Writing by Matthew Mpoke Bigg; Editing by Toni Reinhold)