* Ex-presidential guard Abdel Aziz seeks another 5-year term
* President seen in West as tough on al Qaeda
* Main challengers boycott vote, leaving weakened field (Adds close of polls, opposition comments, details)
By Joe Penney and Kissima Diagana
NOUAKCHOTT, June 21 (Reuters) - Mauritanians voted on Saturday in an election certain to return President Mohamed Ould Abdel Aziz to office for another five years, but he struggled to secure a turnout high enough to give him a convincing mandate amid a poll boycott.
The bulk of the opposition boycotted last year's parliamentary elections - saying the organisers were biased and the process flawed - and talks to try to persuade them to take part in Saturday's vote broke down in April.
That left Abdel Aziz - an ally of Western powers in the fight against al Qaeda-linked Islamists in West Africa - with no major challengers to his rule in the desert nation, which straddles black and Arab Africa.
Analysts said the incumbent's main challenge was to persuade enough voters to take part in the presidential vote and give him a strong mandate.
The election got off to a slow start, with only a trickle of people voting at polling stations in the capital Nouakchott in the first hours of the poll.
Elections commission representatives told Reuters that overall turnout was just 37 percent at 3 p.m. (1500 GMT). But participation appeared to pick up in the early evening.
"It was hot early in the day so I came late to vote," voter Khady Gaye told Reuters. "I voted for Aziz because he started many projects and he should get the chance to finish them. I pity those who boycotted the election."
A Reuters journalist recorded turnout ranging between 36 and 63 percent at several polling stations in the capital slightly before they closed at 7 p.m. (1900 GMT). Poll workers began counting ballots as soon as voting ended.
"Anyone can see that these (opposition) parties are void of any content and no longer even have a political role to play, reflecting the level of their leaders," Abdel Aziz, a former head of the presidential guard, told reporters after voting.
But opposition representatives were calling the boycott a success even before polls closed.
"The population responded massively to our call to boycott an election we did everything to ensure would follow the rules but which the regime stubbornly organised unilaterally," opposition figure Cheikh Sid'Ahmed Ould Babamine told journalists.
Mauritania's 2009 election saw 10 candidates vying for the presidency. In 2007, there were 19. But the boycott of this year's poll thinned out the field of challengers, leaving the country's 1.3 million voters with just a handful to choose from.
"In a truly democratic country, a president is logically not elected with turnout of 40 percent. (Abdel Aziz) must seek a compromise," said Idoumou Ould Mohamed Lemine, another member of the opposition.
The president's four remaining rivals are former government minister Boidel Ould Houmeid; Ibrahima Sarr, a challenger from the 2009 vote; Mint Moulaye Idriss, an administrator at Mauritania's national press agency and the only woman in the race; and anti-slavery campaigner Biram Ould Abeid.
Mauritania officially abolished slavery in 1980, but human rights experts say it remains one of the few countries in the world where the practice still exists.
"Like Nelson Mandela in South Africa who brought hope to the blacks and like Barack Obama in the USA, I will be the first black slave descendant president in Mauritania," Ould Abeid said after he voted.
The country has reserves of iron ore, copper and gold and is trying to boost investor interest in its oil and gas. However, it has long been plagued by political instability and military coups.
Abdel Aziz came to power in August 2008 when he ousted President Sidi Mohamed Ould Cheikh Abdellahi, the country's first democratically elected president, whose short stint as leader was undone by fighting within his own party.
He then won a 5-year term in a 2009 poll that was heavily criticised by the opposition, some of whom still do not recognise the legitimacy of his election.
Western nations soon re-engaged with Mauritania's military, which has taken a strong stand against Islamist groups in the country and neighbouring Mali. (Writing by Joe Bavier; Editing by Andrew Roche)