The arrests took place as protestors approached a meeting point in a square where a police contingent waited for them.
Police spokesman Francisco Noticia confirmed the arrests, while an organiser said the marchers had come under attack from another group and more than 100 were unaccounted for.
“This group of youths, numbering about a dozen people, tried to disturb public order, so we had to arrest them,” Noticia told AFP, adding that a policeman had been injured in the attack.
Police also detained two Angolan journalists working for Portuguese television RTP and Voice of America. It is unclear if they have been released.
The youths of the Student Revolutionary Movement wore T-shirts with slogans like “Freedom and Democracy” and “32 years is too much” in an apparent jibe at current President Jose Eduardo dos Santos, who has been in power for over 32 years.
“A group of 200 people was moving towards the square but another group came and attacked us to make us flee,” said one of the organisers, who identified himself only as Alexandre.
“At the moment over a hundred of our group’s members are missing,” he told AFP in a phone conversation.
The students had planned to demand the removal of an election candidate of the ruling Popular Movement for the Liberation of Angola (MPLA), saying a criminal conviction disqualified him from running.
They also protested the disappearance of two members of their movement, who have been missing since end May.
Angola holds general elections on August 31. The polls will be only the third since the oil-rich nation’s independence from Portugal in 1975.
During the previous vote in 2008, Dos Santos’ MPLA won 80 percent. -Sapa-AFP