There has been a wave of arrests in Equatorial Guinea ahead of an African Union summit in the capital, Malabo, Amnesty International says.
It says police raids have led to the arrest of more than 100 people.
The detentions were intended to prevent pro-democracy protests during the AU summit due to start on Thursday, Amnesty said.
Equatorial Guinea banned protests after uprisings against long-term rulers started in North Africa.
President Teodoro Obiang Nguema has ruled the West African state since 1979.
"President Obiang's government is already among the worst human rights abusers in Africa and the continuing persecution of political opponents is deplorable," Amnesty International's Deputy Director Tawanda Hondora said.
He said there were also reports of detainees being tortured.
Amnesty International said the government was increasingly afraid of pro-democracy protests similar to those that have swept North Africa and the Middle East.
It said that in March, a journalist with state radio was suspended for reporting on the situation in Libya.
A top government official ordered the journalist, Juan Pedro Mendene, to leave the station, it added.
As he was leaving, he was attacked and beaten, the rights group said.
A week later, the director of the state-owned radio station announced that broadcasts were temporarily suspended following orders from a higher authority. No other explanation for the suspension was given, Amnesty said.
It said Equatorial Guinea's security forces have also been arresting illegal migrants, especially in the port city of Bata.
"These indiscriminate raids on migrant communities must be brought to a halt. All undocumented migrants must be treated humanely and in accordance with the law," Mr Hondora said.