Gambian prosecutors have charged eight former spies with conspiracy to murder in the case of an activist whose death in custody sparked a political movement that ousted former leader Yahya Jammeh.

Eight agents of the National Intelligence Agency (NIA) were charged on Wednesday with conspiracy to commit murder and accessory to the facts of murder, police spokesman David Kujabie told AFP on Thursday.

The alleged torture and killing of United Democratic Party (UDP) organiser Solo Sandeng in April 2016 sparked rare protests in the small West African, and encouraged a weak and split political opposition to unite behind a single flagbearer.

That flagbearer, then unknown businessman Adama Barrow, beat Jammeh in December 2016 elections and has since pledged to reform Jammeh's security apparatus, which was regularly accused of kidnap, torture and murder.

The penalty for the charges is the death sentence, but The Gambia under Barrow has pledged to abolish capital punishment.

Nine other former members of the NIA, including its former chief Yankuba Badjie, are already standing trial for their alleged role in Sandeng's murder, which is moving slowly with evidence given by 13 prosecution witnesses.

Jammeh seized power in a bloodless coup in 1994 and ruled the former British colony with an iron fist for 22 years, during which he was accused of rampant corruption and human rights abuses.

His refusal to leave power after losing to Barrow sparked a regional crisis in West Africa, which ended when he agreed to live in exile in Equatorial Guinea.