The rally, organised by a youth movement without the support of any of the main opposition parties, called for the resignation of President Jose Eduardo dos Santos, who has held power in oil-rich Angola for over 32 years.
According to the Portuguese television station RTP, around 200 youths gathered at Luanda's Independence Square at midday. Police intervened when a small group headed towards the presidential palace to demand the release of a protester allegedly arrested earlier on Saturday.
The television station cited one of the rally organisers as saying 12 protesters had been arrested and several injured. RTP added that several Portuguese and Angolan journalists suffered light injuries in the fracas.
The Angolan National Police said four police officers were injured as they tried to persuade the protesters to stay on the authorised route for the protest.
"Against police guidance, some individuals forced the security cordon in an anarchical way, insulting passers-by and police officers and saying they wanted to the head to the palace," the police said in a statement quoted by Angolan state news agency Angop.
"That led to a climate of the violence, which led to the throwing of heavy objects and caused the injuries," it added.
Dos Santos' MPLA party , which won in 2002 won the 27 year-long civil war against Unita and then obtained 82% of the vote in the 2008 general election, has long been accused of mismanaging the country's oil revenues, avoiding public scrutiny and doing too little to fight corruption.
An estimated two-thirds of Angola's 16.5m people live on less than $2 per day in a country which is Africa's second-largest oil producer after Nigeria.
The MPLA is favourite to win a general election next year, but tensions are rising ahead of the ballot after Unita, now the main opposition party, last month accused it of stripping the national elections committee of any real power.