Among those injured are 21 special police officers.

The rioters, among whom were masked members of the Bad Blue Boys group of football supporters, clashed with the police in an attempt to reach St. Mark's Square, where the government is located, throwing rocks, laths and other things, causing major damage to window shops and cars.

Between 800 and 1,000 protesters headed towards a police cordon, with the intention to cause riots and act violently towards the police, who were forced to use force, Zagreb police chief Tomislav Buterin told a news conference.

Police pushed 400-500 violent protesters back towards the main Trg Bana Jelacica square and 33 people were injured in the process, including journalists, a photographer of Agence France Presse, whom police took into custody for his safety, it was said.

The riots calmed down around 4 pm.

Among the protesters was not Ivan Pernar, who was arrested twice before for organising an anti-government rally in St. Mark's Square via Facebook.

Deputy Police Director Drazen Vitez thanked the organisers of the veterans' rally for protesting in a peaceful and dignified manner, for attempting to stop the rioters from attacking the police and for standing between the police cordon and the rioters.

Police estimated that up to 15,000 people rallied in the main square today.

Croatian Television said the hooligans attacked and lightly injured one of its crews.

A protest rally was also organised in the eastern city of Osijek, also via Facebook.

A photographer with Vecernji List daily, Davor Javorovic, has reported he was attacked while shooting the rally, while the ruling Croatian Democratic Union party has reported that the entrance to their premises was damaged, county police spokesman Marko Caleta said.

He added several hundred people were at the rally and that no one was arrested.