BRAZZAVILE (Reuters) - A former army chief and candidate in Congo Republic's presidential election earlier this year was arrested overnight, his opposition party said on Wednesday.

Authorities did not immediately comment on why they had detained General Jean-Marie Michel Mokoko, 69, a power broker in the former French colony's 1990s civil war.

The government of President Denis Sassou Nguesso has accused Mokoko, a one-time security advisor to the president, of involvement in an alleged coup attempt in 2007. Mokoko's supporters say the president is trying to stifle dissent.

"This is clearly a desire to harm the general, who stood up to Sassou during the last presidential election," spokesman for the l'IDC-FROCAD party Guy Romain Kinfouissia said.

"It is just another demonstration of force to muzzle the opposition," he said.

Sassou Nguesso has ruled the oil producing central African nation for all but five of the past 37 years and won a disputed election in March. Most opposition parties boycotted the vote, although Mokoko ran and came third.

Last year he held a referendum on constitutional changes allowing him to seek a third term in power, as several other African leaders have done, leading to protests in which at least 18 people were killed by security forces.

(Reporting by Philon Bondenga; Writing by Tim Cocks; Editing by Janet Lawrence)