Police in The Gambia say they have discovered the body of opposition activist Solo Sandeng, who died in custody during former President Yahya Jammeh's rule last year.

Sandeng led a peaceful opposition march for political reform on April 14, 2016, but died in detention shortly after his arrest by state security agents, unleashing a wave of anger among many in the country who voted out Jammeh eight months later.

His body was found near the western coastal village of Tanji and will undergo forensic investigation at a hospital in the capital, Banjul, police spokesman Foday Conta said on Thursday, adding that his remains were exhumed on Saturday.

The police discovered the burial site based on testimonies by detained national intelligence agency officials, who had worked under Jammeh, according to Conta.

Saikou Omar Jeng, who was the former director of operations at the notorious National Intelligence Agency (NIA), led police to a grave in Tanje, a small fishing village around 33km from the capital, Banjul, police said.

"I can confirm that the remains of Solo Sandeng are currently lying at the Edward Francis Small Teaching Hospital in Banjul," a source at Banjul police headquarters told the AFP news agency.

"Jeng led a team comprising police investigators, medical officers and the late Solo's family members to the latter's grave in Tanje," the source added, specifying that Sandeng's father and brother were those present.

Last June, Jeng admitted that Sandeng had "unfortunately lost his life" during the "process of arrest and detention", but a new post-mortem will be carried out.

Sandeng's death sparked protests by his party, the United Democratic Party, and is seen as a key factor in the formation of an unprecedented coalition of opposition parties that ousted Jammeh from power.

Jammeh provoked outrage by responding to UN calls for a probe into the cause of Sandeng's death with the words: "This time, there is only one dead and they want investigations? I will not," speaking to magazine Jeune Afrique.

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Jammeh, who ruled The Gambia for 22 years, is widely accused of human rights violations, including torture and arbitrary arrests.

The former president caused weeks of political impasse by refusing to accept the result of a December 2016 presidential election which saw him lose power to Adama Barrow.

After weeks of regional pressure and the threat of arrest by West African troops that had entered The Gambia, Jammeh eventually conceded defeat and went into exile in Equatorial Guinea.

Barrow took power on January 19.