Chelsea and Manchester City supporters clashed with police in Porto on Thursday night.
Porto Mayor Rui Morera said Sky Sports News The flashpoints were near the Dover River, where supporters were drinking.
No arrests were made but one supporter went to the hospital with minor injuries.
The bars in Porto have reopened as the city comes out of lockdown, but they must be closed by 10:30 pm to comply with the city’s strict curfew.
Thousands more Chelsea and City fans arrive in Porto on Friday and Saturday ahead of this week’s Champions League final between the two sides.
Each club will have around 1,000,000 supporters at the Estadio do Dragao for a game in front of 1, 50,000 spectators.
While some fans have made their way to Porto, the majority arrive on chartered flights on Saturday.