(Updates with top AU diplomat's comments, paragraphs 9, 13-14)
By Hachem Sidi Salem and Gabriela Matthews
NOUAKCHOTT, Aug 24 (Reuters) - The military coup that toppled Mauritanian President Sidi Mohamed Ould Cheikh Abdallahi this month threatens the future of democracy in Africa if left unchallenged, the deposed president's son said on Sunday.
In an interview with Reuters in Nouakchott, Abdallahi's youngest son Ahmed said he believed his father would eventually be restored to power by pressure from within Mauritania and from the international community, which has cut millions of dollars of aid following the Aug. 6 coup in the West Saharan state.
"I've no doubt that he'll return, it could take 15 days, it could take a month, but there's no doubt that this country can't live without economic aid," Ahmed Ould Sidi Ould Cheikh Abdallahi said.
He complained that the coup leaders were preventing members of Abdallahi's family from visiting the toppled president, who has been kept in detention since the latest military takeover in the country that became Africa's newest oil producer in 2006.
President Abdallahi, Mauritania's first freely elected head of state, was deposed by officers led by the chief of his own presidential guard, General Mohamed Ould Abdel Aziz, barely 15 months after he took office after winning elections.
That vote was organised after a 2005 coup -- also instigated by Abdel Aziz -- which toppled authoritarian ruler Maaouya Ould Sid'Ahmed Taya, who had taken power in a coup 20 years earlier.
The Aug. 6 coup followed Abdallahi's sacking of senior military officers, including Abdel Aziz, who were widely seen as supporting the president's opponents. Abdel Aziz says he took over because Abdallahi had shown poor leadership.
The African Union has suspended Mauritania after the coup, which has also been strongly condemned by the United Nations, the European Union and the United States.
The AU's top permanent official, Jean Ping, flew into Mauritania later on Sunday to hold talks with the coup leaders and with supporters of Abdallahi. Ping told Reuters he had come "to listen" and had not brought any "miracle solutions".
Abdallahi's son Ahmed called the coup and his father's detention "an act of piracy".
"We're looking at a military coup d'etat in the full sense of the word," he said.
"The future of democracy in Africa is at stake. If this coup is allowed to stand -- and it won't be -- then all the democracies in Africa are entirely threatened," he added.
Ping, who heads the AU Commission, was expected to press the coup leaders to release Abdallahi, but he declined to spell out beforehand exactly what message he would convey to them.
"We haven't come to apply threats, we've come to discuss and seek solutions," he said on his arrival in Nouakchott.
Abdallahi's son Ahmed said his father was not being allowed either telephone calls or visits by members of his family.
"Speaking as a son, I know that a president shouldn't be treated like that, it's against common law because even terrorists and common prisoners can be visited by their family," he said, adding he had contacted the International Committee of the Red Cross about gaining access to his father.
"I think he's in good health," he added, citing reports from the French foreign ministry and media. France's ambassador was allowed to see the detained Abdallahi on Thursday.
The coup leaders are also holding former Prime Minister Yahya Ould Ahmed El Waghef, who was deposed along with Abdallahi, after El Waghef last week led a big protest march against the military coup.
Thousands of Mauritanians took part on Wednesday in the largest public protest ever seen in the Islamic state, which straddles black and Arab Africa and besides oil has rich fisheries stocks and also produces iron ore and gold.
But Abdel Aziz's backers have organised a series of rallies in support of the coup, and a majority of parliamentarians have thrown their weight behind the junta. Abdallahi's son said the reduction of foreign aid could worsen the situation of the poor inhabitants, who had already been feeling the squeeze of soaring food and fuel prices.
"I think the country will now unfortunately live through some difficult times," he said. (For full Reuters Africa coverage and to have your say on the top issues, visit: africa.reuters.com/) (Writing by Pascal Fletcher)