Earlier in the day, Ben Sekou Sylla, president of the national council for civil society organisations, said: "Hundreds of people have been arrested at night over the last few days, generally by order of the ruling party's leaders."
Those picked up were being taken to military camps or police stations, he said
More than 120 people have been killed, almost all of them civilians, in protests since the start of the year.
Human-rights groups accuse security forces of firing on unarmed crowds, beating protestors, looting and raping civilians.
"We have counted 278 arrests since the start of martial law," said Mohamed Diane, secretary-general of the opposition Assembly of the People of Guinea.
Thierno Maadjou Sow, president of Guinea's Human Rights League, said his organisation had been informed of hundreds of arrests, but was still compiling its own figures.
It was also investigating reports of the death of 22 prisoners in Nzerekore, in Guinea's south-eastern border area with Liberia, Sierra Leone and Ivory Coast.
Guinea's unions, who called a general strike in January which sparked the current unrest, were refusing to enter into talks aimed at ending the unrest until martial law is lifted.