* Loser Ping challenges election result in top court
* Challenge focused on President Bongo's stronghold
* African Union mission had been due on Friday
* France urges recount, EU says voting anomalies (Adds quotes, context)
By Gerauds Wilfried Obangome
LIBREVILLE, Sept 8 (Reuters) - Gabon's opposition leader lodged a constitutional court challenge on Thursday against a presidential election he narrowly lost, hoping to overturn a result whose validity has been questioned at home and abroad.
Former foreign minister Jean Ping lost the Aug. 27 election to President Ali Bongo by fewer than 6,000 votes, an outcome that sparked days of riots in which at least six people were killed.
Ping's spokesman said in a statement he would seek a recount in the province of Haut-Ogooue, a stronghold of the Bongo family, who have ruled the central African oil-producing nation of 1.8 million for nearly half a century.
The poll and its aftermath have shone a rare and unwelcome international spotlight on Gabon, a former French colony where petrodollars, invested by foreign firms including Total TOTF.PA and Royal Dutch Shell (LON:RDSa)
France, which still has a military base the country, reiterated calls for a recount, a foreign ministry spokesman saying a "transparent, impartial examination of the results" was needed to resolve the political crisis.
Bongo has said only the court can consider that request, while Foreign Minister Emmanuel Issoze Ngondet said an African Union (AU) mediation mission, due to arrive in the now becalmed capital Libreville on Friday, had been postponed until further notice.
The mission's head, Chad's President Idriss Deby - who took power in 1990 and is one of Africa's longest serving rulers - was "tired" after attending a summit in China, the minister said.
Opposition parties in Africa frequently dispute elections citing fraud and, while it is unusual for foreign governments to press for further scrutiny after an election has been declared, results are rarely overturned.
Ping has also said he says he has little faith in the court, his only legal avenue for redress and which he says is tied to the government.
ANOMALIES
Criticism of the poll has focused on Haut-Ogooue, where results showed 95.46 percent of voters backed Bongo on a turnout of 99.9 percent, more than double the participation rate of other regions.
The president, who has in turn accused the opposition of cheating, was re-elected after coming to power in 2009 on the death of his father, Omar Bongo, who had ruled for 42 years.
Ping says the Haut-Ogooue numbers were inflated to give Bongo victory, and the European Union has also reported anomalies.
Sarah Crozier, EU election monitor spokesman, said on Thursday the official turnout for Haut-Ogooue indicated just 47 abstentions out of more than 71,000 registered voters.
Election uncertainty may complicate Gabon's adjustment to lower oil prices and that could have implications for the country's credit profile, Fitch Ratings agency said on Thursday.
Gabon produces 200,000 barrels of oil per day.
Critics of Bongo say he has not done enough to redistribute the wealth of that production beyond a small elite.
Bongo has accused the opposition of cheating in its turn and said on Wednesday he would ask the constitutional court to investigate irregularities in Ping's stronghold and elsewhere.
Ping, a former African Union Commission chairman, was an ally of Omar Bongo but fell out with his son and resigned from the ruling party in 2014. (Writing by Matthew Mpoke Bigg; editing by John Stonestreet)