Some 500 youths, mainly teenagers, have raided a popular beach near Lisbon spreading panic among beachgoers and mugging a large number of people.
Carcavelos beach, 15km (10 miles) west of the Portuguese capital, was crowded on Friday because the country was celebrating a national holiday.
Policemen in riot gear were confronted by gangs and struggled to restore calm, making four arrests.
The attackers are believed to come from some of the capital's poorest suburbs.
"Small groups of youngsters aged 12 to 20 assembled and robbed en masse those they found on the beach," a local cafe owner, Helder Gabriel, told AFP news agency.
A small number of police officers from the nearby town of Cascais initially tried to disperse the mob, but were forced to call reinforcements because the gangs were refusing to leave the beach.
Five people, including two police officers, suffered minor injuries.
Cascais municipal authorities believe the youngsters came from poorer suburbs of the capital.