Guineans took to the streets en masse on Tuesday, in the latest round of mass anti-government protests to hit the fragile West African state.
Around one million people protested against embattled President Alpha Conde in the capital Conakry, opposition MP Fode Oussouba said.
AFP could not independently verify the figure, however.
The poor former French colony country of some 13 million has seen rolling demonstrations since mid-October over suspicions that the 81-year-old president is maneuvering to seek a third term in office.
At least 20 civilians have been killed since protests began, and one gendarme has also been killed.
Scores of people have also been arrested and detained in the unrest.
There were no reports of violence on Tuesday, however, which also saw thousands of people protest in regional cities in the center and north of the country, according to witnesses.
"Today, our thoughts are with the dead, with our friends who are in court," said Abdourahmane Sanoh, the coordinator of the National Front for the Defence of the Constitution (FNDC), an alliance of opposition groups.
Sanoh was temporarily freed from prison last month, pending an appeal, after originally being jailed for his role in staging demonstrations.
Conde, whose second term ends next year, launched constitutional consultations in September, saying the former French colony's basic law "concentrates corporate interests" and needed reform.
But his adversaries say the president will try to push through an amendment allowing him to seek a third term in elections due in 2020. He has neither confirmed nor denied his intentions.