CONAKRY (Reuters) - Guinea has indicted a gendarme officer on rape charges related to a stadium massacre of pro-democracy protesters by forces linked to the military junta in September 2009.

Rights groups have criticized President Alpha Conde, elected in 2010 in Guinea's first democratic handover of power since independence from France in 1958, for not moving fast enough to bring those responsible to justice.

About six people have so far been charged in relation to the attack, though none have been tried. The officer is the first in the case to be indicted for rape.

"A gendarme officer has been formally indicted on rape charges after he was identified by one of his victims," said Thierno Maadjou Sow, head of the Guinea chapter of the International Federation for Human Rights.

"The rape took place during the September 28 massacre," he said. "The indictment is a first in Guinea. It will certainly allow us to move forward on this issue."

A Guinean official confirmed the charges but declined to identify the officer.

More than 150 people were shot, stabbed, bludgeoned or trampled to death after pro-democracy demonstrators gathered at a stadium in the minerals-rich nation's capital for a rally.

A U.N.-led probe concluded the abuses likely constituted crimes against humanity.

Corinne Dufka, a senior researcher at Human Rights Watch who interviewed over 20 rape victims after the incident, said the indictment offered a glimmer of hope.

"These women and girls were subjected to sexual violence of particular brutality, including individual and gang rape with batons, rifle butts, and bayonets. Indeed at least four women were murdered after being raped," she told Reuters.

"We believe most of these assaults were carried out by members of the Presidential Guard. We salute the progress this indictment represents and hope others will follow," she added.

Several officials of the former military junta that ruled Guinea for two years after a 2008 coup have been charged with crimes relating to the massacre, including Lieutenant Aboubacar Sidiki Diakite, then the head of the Presidential Guard.

(Reporting by Saliou Samb; Additional reporting by David Lewis; Writing by Bate Felix; Editing by Raissa Kasolowsky)