Four men were arrested following clashes at an opposition Democratic Party (PD) rally in the Albanian capital Tirana, after Molotov cocktails were thrown at officers guarding the prime minister's office, police said.
Seven others face investigation without detention, according to authorities.
Among those under investigation is Ervin Minarolli, PD's organisational secretary, who police said failed to identify persons responsible for ensuring the smooth conduct of the rally as required under Albanian law on public gatherings.
Monday's protest took place on the central Martyrs of the People boulevard, outside Prime Minister Edi Rama's residence, around 6 pm. Police said some participants threw petrol bombs at officers deployed to maintain public order and security.
Three police officers injured in the incident received hospital treatment and are out of danger, according to authorities. A photographer and one protester were also among those injured.
Police said an investigative team is continuing work to identify others who may have committed illegal acts during the rally. A 16-year-old minor is among those under investigation.
Case materials were referred to Tirana Prosecutor's Office for further action, police said.
The opposition PD party regularly organises protests against the government led by Rama's Socialist Party (PS), which has been in power since 2013.
