Unidentified attackers threw petrol bombs at a police station in the Portuguese city of Setubal and torched cars in the capital Lisbon, hours after a protest against alleged racially motivated police violence ended in clashes.

Police said in a statement that three petrol bombs caused damage to the station in the early hours of Tuesday morning but nobody was hurt.

The statement added that their investigation could not establish any links to a rally in central Lisbon on Monday night, which followed a police raid on a predominantly black neighbourhood in Seixal, on the outskirts of the capital south of the Tagus River.

On Sunday, police responded to a call about a brawl between two women and entered the "Jamaica" neighbourhood in Seixal, where mostly young residents allegedly greeted them with a hail of stones.

A video published online showed police officers beating several black people. Four local residents and one police officer suffered minor injuries.

One person was arrested and later released.

A young woman, who asked not to be named, told Reuters news agency she had filmed the incident and denied stones were thrown.

"We want justice and we hope they are charged," she said.

Lisbon's public prosecutor's office said it would investigate the incident.

Julieta Luvunga, 53, who is originally from Angola and lives in the neighbourhood, said she was taken to the hospital after being hit by the police and falling unconscious.

"I never thought police would hit me. We see this happening in other countries, like the United States, but I never thought it would happen here," she told Reuters.

Rights group SOS Racismo said the police response was unjustified.

About 200 mostly black protesters blocked Lisbon's Avenida da Liberdade thoroughfare on Monday night, chanting "Down with racism".

Stones were thrown at police officers who dispersed the rally.

Police said on Tuesday that one person was arrested on suspicion of torching four vehicles and a dozen garbage cans in greater Lisbon's Odivelas district.

There are several poor neighbourhoods on the outskirts of Lisbon where immigrants, mostly from Portugal's former African colonies, live.

Race-related violence is rare in Portugal but in one incident in Lisbon in 2015, 18 police officers were accused of crimes motivated by racism. Seventeen of them are now on trial.