Soldiers fired into a crowded stadium where demonstrators had gathered to protest against the West African nation's coup leader Monday, killing at least 10 people, an Associated Press reporter said.
Opposition parties had organized the protest in the capital's main football stadium, which drew some 50,000 people. Demonstrators chanted "We want true democracy."
Red-bereted soldiers from the presidential guard later entered the stadium and fired into the crowd. An AP reporter counted at least 10 bodies.
Tensions have risen amid rumors that military leader Capt. Moussa "Dadis" Camara may run in presidential elections scheduled for Jan. 31.
Camara came to power in a coup last December, hours after longtime dictator Lansana Conte died. Camara initially said he would not run in the election, but has recently said he has the right to run if he chooses.
The international community already has cut off aid and frozen Guinea's membership in the African Union since the coup.
"We are even prepared to pay with our lives to ensure that the military does not rule our country any more," said former prime minister Sidya Toure, who was arrested Monday as he left the scene. Authorities also arrested a second ex-prime minister, Cellou Diallo.
Demonstrations against Camara have grown but none have been as violent as Monday's demonstration. On Aug. 27, police fired tear gas to break up a protest in the capital. And last Thursday, tens of thousands of residents in a town north of Conakry took to the streets with no serious incidents.
Camara has denounced the demonstrations against his presumed candidacy and accused political party leaders of being behind them.
"These politicians send your children onto the streets, while theirs are overseas in plush houses," he said last month.
Hardly anyone had heard of Camara, an army captain in his 40s, until Dec. 23, when his men broke down the glass doors of the state TV station. He announced that the constitution had been dissolved and that the country was now under the rule of a military junta.
Since winning independence half a century ago from France, Guinea has been pillaged by its ruling elite. Its people are among the world's poorest, even though its soil has diamonds, gold, iron and half the world's reserves of the raw material used to make aluminum.