DAKAR (Reuters) - Gambia sentenced a prominent lawyer and 18 of his supporters to three years in jail on Wednesday for "unlawful assembly" and for holding a protest without a permit, a court document reviewed by Reuters showed.

Ousainou Darboe was arrested after taking part in a small protest near the capital Banjul, along with other senior members of the United Democratic Party (UDP). Both the United Nations and the United States condemned the arrests.

Justice Eunice Dada Oshim convicted them for unlawful assembly, for "riotously interfering with vehicles", holding a procession without a permit and disobeying an order to disperse.

The total sentences came to eight years, but the judge ruled that they should run concurrently.

Protests are rare in Gambia, a tiny territory along the banks of a river that is almost encircled by Senegal.

President Yahya Jammeh has ruled for two decades since seizing power in a bloodless coup. But rights groups say he has been cracking down on opposition figures in recent months with an eye on December's election, where he will seek his fifth term, after he scrapped term limits.

"The sentencing of senior opposition figures such as Ousainou Darboe is the latest in a continuous chain of violations committed against those who dare to speak out in Gambia," Stephen Cockburn, Deputy Regional head of Amnesty International in West and Central Africa, said in a statement.

"Imprisoning opposition leaders ... not only fragrantly violates their human rights but is also likely to enflame an already tense situation".

(Writing by Tim Cocks; Editing by Hugh Lawson and Bernard Orr)