A hundred members of the opposition Movement for the Rebirth of Cameroon (MRC) were freed Thursday following their arrest during demonstrations last week, the party told AFP.
However MRC vice president Mamadou Mota, who was also detained on June 1, remains in detention, the same opposition party source said.
"Today 100 of our members, arrested on Saturday... have been freed," said lawyer Christopher Ndong, secretary general of the MRC.
In all, 351 MRC supporters were arrested during protests which took place in several towns throughout the country, including Nkongsamba, in the west, and Yaounde, according to the party's figures.
"There are still 251 people incarcerated in prison cells," said Ndong.
At least 30 of those arrested, including Mota, have been transferred to Yaounde's main prison.
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The prison also houses MRC head Maurice Kamto, the country's main opposition figure who has been detained since January.
The MRC has been organising demonstrations since the October 2018 presidential election. According to official results, Kamto came second but the MRC says the vote was rigged in favour of President Paul Biya, who has been in power for 36 years.
Saturday's protests called for the release of Kamto and other party supporters who were arrested after another protest in January.
Such protest marches are banned by Cameroon.
Video footage posted on social media sites Thursday show MRC militants claiming, after their release, that they were tortured while in detention.
The use of torture in police station is a frequent occurrence according to many sources.
In February Kamto and other opposition supporters appeared before a military tribunal in Yaounde accused of "insurrection, hostility to the homeland (and) rebellion", offences which carry a possible death penalty.
Their lawyers have appealed to the UN working group on arbitrary detention over the arrests.