United Democratic Party (UDP) chief Ousainou Darboe, a human rights lawyer, was seen being hauled away by police with three other party leaders after beginning a protest march from his residence just outside the capital of Banjul.
Gambian security forces armed with assault rifles fired tear gas at the protesters, according to eyewitnesses.
Around 150 supporters had joined Darboe to call for justice in the case of UDP organising secretary Solo Sandeng, who died in custody on Thursday.
The UDP confirmed Sandeng's death to AFP today morning, while Darboe told a gathering of activists prior to his arrest that two female protesters were also in a coma but remained in detention.
"We have received some reports that Solo Sandeng died in detention. We understand he died shortly after his arrest for participating in what we've been told by eyewitnesses was a peaceful protest," said Amnesty International west Africa researcher Sabrina Mahtani.
Gambia's information minister did not immediately respond to a call for comment.
President Jammeh was out of the country when Thursday's protest happened, attending a summit of leaders from the world's Muslim countries in Istanbul.
A military officer and former wrestler, he has ruled the tiny west African country with an iron fist since he seized power in a coup in 1994, and is regularly accused of sanctioning a catalogue of human rights abuses.
"We are concerned with the election period coming up that there will be a further crackdown on fundamental human rights," she said.
