Authorities in Gambia have detained the former head of the National Intelligence Agency, a body human rights groups say tortured and killed perceived opponents of former leader Yahya Jammeh, who went into exile last month after losing an election.

The former director general of the agency, Yankuba Badgie, and its ex-director of operations, Sheikh Omar Jeng, were arrested on Monday, police spokesman Foday Conta said on Tuesday.

Jammeh set up the agency the year after he seized power in a coup in 1994 and it gained a reputation as the state's most feared institution.

The government of President Adama Barrow, who defeated Jammeh in an election in December, stripped the agency of its powers of arrest, changed its name to the State Intelligence Agency and pledged to reform it further.

Monday's arrests are the first of senior Gambian officials since Jammeh went into exile in Equatorial Guinea under pressure from regional leaders who deployed troops to Gambia in a bid to force him to leave power.

Gambian police said this week they had arrested 51 people in a former Jammeh stronghold for harassing Barrow's followers.