* Five candidates challenge Congo vote results

* Complain Sassou-Nguesso re-elected fraudulently

By Christian Tsoumou

BRAZZAVILLE, July 23 (Reuters) - Five candidates who lost an election in Congo Republic have now filed an official challenge to incumbent President Denis Sassou-Nguesso's victory in a poll last week, a candidate said on Thursday.

Congo's Constitutional Court has two weeks to analyse the complaints of fraud made against the July 12 election, which the government said Sassou-Nguesso won with 78.6 percent of the vote, securing himself another seven years in power.

"We have lodged our complaint with the Constitutional Court for the same reasons as the others -- that is to say, massive fraud," opposition candidate Clement Mierassa told Reuters.

Mierassa joins fellow opposition candidates Mathias Dzon, Bonaventure Mizidy and Guy Romain Kinfoussia and independent candidate Jean Francois Tchibinda Kouangou in demanding that the court annuls Sassou-Nguesso's victory due to fraud.

Sassou-Nguesso has been in and out of power since a 1979 coup, losing elections in 1992 before sweeping back into power in a war that destroyed much of the capital in 1997. Most of his rivals were banned from or boycotted the last poll in 2002.

"There was fraud in the electoral lists," Mierassa said, highlighting the case of Kouilou, in the south of Congo, where he said the 63,000 registered voters were far too high for a population of 92,000 people.

Observers from the African Union and a grouping of central African nations have said that the poll was transparent.

The European Union did not send observers but was highly critical of the preparations of the poll. Meanwhile, a local rights group called it blemished by fraud and uncredible.

Herve Ambroise Malonga, Dzon's lawyer, said the poll could not have been credible as the government had provided three different figures for the number of people eligible to vote.

"What my client saw was not a vote but a stuffing of ballot ... that is why he has asked me to lodge a request to annul the election, which has brought shame on Congo," Malonga said. (Writing by David Lewis; Editing by Diana Abdallah)

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