"The mood on the street is hardening against the junta," Richard Moncrieff, the West Africa project director of the International Crisis Group, said in a press release.
"Worse trouble is likely unless combined domestic and international pressure is applied to force the soldiers from power."
On Saturday, West African heads of state will meet to discuss sanctions and possibly call for an international inquiry into the September 28 bloodbath.
Meanwhile, a rally in support of Captain Moussa Dadis Camara, the country's leader, was scheduled for Saturday.
Mamadi Kaba, a spokesman for the Guinean branch of African human rights watchdog Raddho, told the news agency AFP: "Tension prevails. There will be a rally Saturday in Conakry in support of the junta, as an act of defiance against the international community."
"[The rally] is considered an insult by a part of the population. I am afraid the situation will get out of hand," Kaba said.
Three ministers have resigned this week in protest against the bloody crackdown on the opposition.
Justin Morel, the information minister, said in his resignation letter: "My conscience has remained tormented, my heart disturbed, and my sense of reasoning has told me that I no longer have any reason to continue to head this ministry and neither do I have the moral force to be the spokesman of the government after these horrible killings."
Camara seized power in a coup in December, hours after the death of Lansana Conte, the president. He vowed to hold elections and not stand for himself, but there have been widespread rumours that he is aiming to stay in power.
The African Union has given Camara's government until midnight on Saturday to pledge in writing that it will not field a candidate for January's presidential elections.