* Anti-slavery campaigner Abeid rejects victory claim
* Mohamed Ould Abdel Aziz stepping down after decade at helm (Updates with victory declared)
By Kissima Diagana
NOUAKCHOTT, June 23 (Reuters) - Mauritania's government, which has positioned itself as an ally of the West against Islamist militants, declared victory on Sunday in the country's presidential election but opposition candidates said they could contest the results.
Data from the electoral earlier showed the ruling party's Mohamed Ould Ghazouani comfortably ahead after Saturday's vote, taking 50.41% of the ballot with more than half votes counted, a clear but still far from unassailable lead.
If no one gets more than half the votes, the election goes to a second round.
"Congratulations to president-elect Mohamed Ould Ghazouani for the trust the people have shown him. We wish him all success in his work," Communications Minister Sidi Mohamed Ould Maham wrote in Arabic.
Ghazouani has campaigned on continuing economic and security progress made under outgoing president Mohamed Ould Abdel Aziz, who took the helm in a 2008 coup.
Under the leadership of the 62-year-old president, the economy has grown and will receive an extra boost when a large offshore gas field starts producing early next decade.
At a news conference, however, opposition candidates said they would contest the results if the ruling party, which they said had no right to proclaim on the results, won outright.
"This seems like a coup d'etat," Ghazouani's nearest rival, Biram Dah Abeid, representing himself and the other opposition leaders, said. "We are united and will lead the contestation (of the results)."
Abeid, a prominent black Mauritanian slavery campaigner, has got 18.72% so far, the figures showed on Sunday. Mohamed Ould Boubacar, who is backed by Mauritania's biggest Islamist party, has 18.13%.
None of the three remaining candidates has more than 10%.
The election was the first in the sparsely populated Saharan nation's history since independence from France in 1960 to choose a successor to a democratically elected president.
Abdel Aziz surprised many of his compatriots and international observers by stepping aside after serving the maximum two five-year elected terms in Mauritania, a country of fewer than 5 million people comprising a large chunk of the western Sahara Desert.
His decision bucked a trend in which African leaders, including in Rwanda and Congo Republic, have changed or abolished term limits to cling to power.
About 850,000 votes out of 1.5 million had been counted in the latest public electoral commission figures.
Despite his economic record, Abdel Aziz has been criticised for not facing up to the country's most searing injustice: The persistence of slavery. Tens of thousands of black Mauritanians still live as domestic slaves, rights groups say, usually to lighter-skinned masters of Arab or Berber descent.
That is despite the practice being abolished in 1981 and criminalised in 2007, the year before he took power.
He has made pronouncements denying slavery is widespread.
Abeid, himself a descendent of slaves, has campaigned partly on this platform. He and other opposition leaders also sought to tap into youth anger at high unemployment. (Writing by Tim Cocks Editing by Louise Heavens, Susan Fenton and Alison Williams)
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