* Mauritanians vote for first successor to democratic president
* Abdel Aziz could maintain influence behind the scenes
* Opposition concerned about fraud (Adds comments by outgoing president, fraud concerns by opposition candidates)
By Kissima Diagana
NOUAKCHOTT, June 22 (Reuters) - For the first time since Mauritania's independence, its citizens voted on Saturday for a successor to a democratically-elected president, though a government insider campaigning on a message of continuity is heavily tipped to win.
Polls opened at 7 a.m. (0700 GMT) in the election to replace President Mohammed Ould Abdel Aziz, 62, who since seizing power in a 2008 coup has positioned himself as an ally of the West in the fight against Islamist militants.
Located on the northwest African coast and bordered to the east by the Sahara Desert, the country of fewer than 5 million people gained independence from colonial power France in 1960.
Abdel Aziz is stepping aside after serving the maximum two five-year elected terms and has thrown his support behind 62-year-old Mohamed Ould Ghazouani, a former general and defence minister.
Abdel Aziz could maintain significant influence behind the scenes. He said on Thursday that he had not ruled out running again in five years when his term limits would reset.
In the capital Nouakchott's wealthy Tevragh Zeina neighbourhood, more than 100 voters queued calmly in a dusty school courtyard to cast their ballots.
"I thank God that everything is going well," said Abderrahime Sidi, 35, after voting. "We hope that the results will be consistent with the expectations of all Mauritanians."
Gilles Yabi, the founder of West African think tank WATHI, said Ghazouani was the clear favourite and would likely continue to rule in Abdel Aziz's mould, but that he could still surprise.
"Ghazouani is someone who is very discreet. It could well happen that the change is not merely cosmetic," Yabi said.
Five candidates besides Ghazouani are on the ballot. Former prime minister Sidi Mohamed Ould Boubacar, who is backed by Mauritania's biggest Islamist party, has drawn large crowds on the campaign trail and is considered Ghazouani's main rival.
Boubacar and another opposition candidate, Mohamed Ould Maouloud, told reporters at their polling places that they were concerned about potential fraud. They have pointed to the absence of international observers and the printing of ballot papers by a company with ties to the ruling party.
Election authorities said there were no political considerations in awarding the contract.
ECONOMY AND SECURITY
Ghazouani has campaigned on the progress made under Abdel Aziz on economic and security issues. Gross domestic product is growing and will receive an extra boost when a large offshore gas field starts producing early next decade.
Tourists are also starting to return for desert tours after years of staying away following a series of kidnappings in 2009.
In recent years, Mauritania has been spared the attacks by jihadist militants linked to al Qaeda and Islamic State that have devastated other countries in West Africa's Sahel region, including neighbouring Mali and Burkina Faso.
Al Qaeda documents seized from Osama bin Laden's Pakistan hideout in 2011 indicated the group's leaders had discussed a possible peace deal the previous year with Mauritania's government that would involve prisoner releases and payments.
The government denied any such deal existed and has credited its success preventing Islamist attacks to intelligence work and rehabilitation of imprisoned jihadists.
"The security and stability of the country must be the priority in choosing the president we need," Abdel Aziz told reporters after casting his ballot.
The opposition candidates, who include a prominent campaigner against slavery in the country, have tried to tap into dissatisfaction among young people over stagnating salaries and poor health care.
Polls are scheduled to close at 7 p.m. If no candidate receives more than 50% of the vote, the election will go to a second round next month.
(Writing By Aaron Ross, Editing by John Stonestreet and Ros Russell)
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