Angolan police moved in on a radio station and held a journalist as protesters called for the release of a rapper and other political activists, Reporters Without Borders (RSF) said on Thursday.
Police picked up five people on Wednesday including Goncalves Vieira, whose opposition station Radio Despertar frequently urges young people to demonstrate against the regime, a source close to the protest movement said.
"Five other people were arrested during the demonstration" at Independence Square in the capital Luanda, where security forces had been waiting in force for several hours, the source added.
International media watchdog RSF said that Vieira was "held for two hours when he was trying to cover preparations for a peaceful protest" against arbitrary imprisonment and political persecution in Angola."
The police cordon around the radio station intimidated the journalists inside from coming out to report on the demonstration, RSF said.
The ruling People's Movement for the Liberation of Angola (MPLA) meanwhile organised a rally of its youth wing to counter the protest in Independence Square with loud music, leading to a tense stand-off.
Police with dogs then charged the opposition protesters and at least one person was bitten, according to photos of the scene circulating on social networks.
Fifteen people accused of seeking to oust President Jose Eduardo dos Santos have been behind bars since June. The one-time Marxist has held power in the oil-rich southwest African country for more than 35 years.
Those held include rapper Luaty Beirao, who is a figurehead for protest in Angola, and Manuel Nito Alves, who was already jailed for more than two months in 2013 for printing T-shirts with slogans against Dos Santos.
Their arrests caused international protests, but on Wednesday deputy state prosecutor Helder Pita Gros justified their detention on national television.
They were not arrested for their political views, he said, but because they were planning to launch an uprising by students and workers, "with incalculable consequences".
Since March 2011, young people have been calling for mass protests to oust Dos Santos, while the regime has systematically banned demonstrations and police have brutally put them down.
Since the end of a 27-year civil war in 2002, Angola has enjoyed strong economic growth as the second largest oil exporter in Africa after Nigeria, but more than half of the population subsists on less than two dollars a day.