A protest rally under the slogan "Stop Prosecution of Croatian Veterans", organised by the Association of War Veterans 1990-1996 started in Zagreb's central Ban Jelacic Square at 1315 hours on Saturday.
The rally, running 15 minutes behind schedule, brought together about 10,000 people, according to police estimates.
More protesters are arriving at the scene.
Trams are not running, police said adding that one protester was apprehended.
The organiser claim that the event started 15 minutes late because "protesters were stopped at roads leading to the main square. Police however claim that protesters were not stopped on their way to the main square, adding however that "there are police controls".
After observing a minute of silence and playing the national anthem, the present are expected to be addressed by war veterans Petar Janjic, Ilija Ackar, Renato Selj, retired general Zeljko Sacic and singer Marko Perkovic Thompson.
Organisers of the event announced the participation of workers, farmers, fishermen and other citizens in the protest as well, adding that they want to express their dissatisfaction with the way the Croatian government and the State Prosecutor's Office are treating war veterans as well as with the general economic and social situation in the country.
Protesters are carrying banners reading "Serbia is the Aggressor, Croatia is the Victim", "Croatian War Veterans are Heroes", "Stop Prosecution of Croatian Veterans, Take Seks - Give Us Back Our Veterans", "I Love Croatia - Not the EU".
Some 200 veterans gathered today for another protest rally in the southern Croatian coastal town of Sibenik, expressing their dissatisfaction with the government's treatment of veterans and the economic situation in the country.
