The Gambia's opposition leader Ousainou Darboe and 18 others have been jailed for three years for taking part in an unauthorised demonstration.
In April, they were part of a group protesting about the alleged death in custody of an opposition activist.
Rights group Amnesty International described the sentences as part of the "continuing downward spiral for human rights in Gambia".
President Yahya Jammeh has in the past dismissed criticism of his record.
Political tensions are rising in the country in the run-up to elections in December, says the BBC World Service Africa editor James Copnall.
Mr Darboe and many of his supporters from the United Democratic Party (UDP) took to the streets in Serrekunda, near the capital, Banjul, on 16 April, demanding the release of their colleague Solo Sandeng "dead or alive".
He had been arrested, along with other activists, two days earlier, and it was alleged that he had died in custody after being beaten.
Mr Sandeng has not been seen since and Amnesty International says he was killed.
The judge found that Mr Darboe did not have permission for the demonstration and sentenced him, and the 18 others, on six charges relating to this.
Reports from the court says the convicted activists sang the national anthem after they were sentenced.
In a statement, the UDP called the trial a "farce" and described the verdict as a reflection of "a corrupt and discredited effort to arrest, torture and persecute innocent citizens".
In an interview in May, President Jammeh said it was "common" for people to die in detention or while under interrogation.
He said UN chief Ban Ki-moon and Amnesty International could "go to hell" for asking for an investigation.