An opposition activist in The Gambia, Solo Sandeng has died in detention.
Amnesty International, along with the other activists, has called for an inquiry into the death of Solo Sandeng.
Amnesty said another activist Fatoumata Jawara, who was detained with Mr Sandeng during protests on Thursday, is believed to have serious injuries.
Several senior opposition leaders were arrested on Saturday after demanding answers from the authorities.
There has been no comment yet from the Gambian government.
Amnesty International says Mr Sandeng, the National Organising Secretary of the opposition United Democratic Party (UDP), and fellow UDP member Ms Jawara were among a number of people detained after taking part in a peaceful protest on Thursday.
The group said the circumstances surrounding Mr Sandeng’s death are unclear, but that he died shortly after his arrest. They are also deeply concerned for the welfare of Ms Jawara.
“The tragic death in detention of Solo Sandeng must leave no space for impunity. The authorities must conduct an immediate, thorough and independent investigation,” said Sabrina Mahtani, Amnesty International West Africa researcher.
Senior members of the UDP, including the leader Ousainu Darboe, held a news conference earlier on Saturday in which they demanded answers.
Along with around 150 supporters, they then began a protest march, but were swiftly rounded up by Gambia’s security force who eyewitnesses said fired tear gas at the crowd.
Mr Sandeng had been calling for electoral reform when he was held. The protest happened while long-time leader Yahya Jammeh was out of the country.
The West African nation is set to hold presidential elections in December. Opposition groups have called for reforms to enable the elections to be free and fair.
President Jammeh has ruled The Gambia since he came to power in a coup 20 years ago.
last year September, Human Rights Watch brought out a report in which it said Gambian forces routinely committed abuses, and political opponents and critics of the president were regular targets.