By Marcin Goettig and Pawel Florkiewicz
WARSAW (Reuters) - Polish police used rubber bullets on Monday to break up groups of masked far-right youths who threw firecrackers and set fire to cars when a nationalist march through the center of the capital turned violent.
The march is an annual event to commemorate Poland's national independence day, and for the third year in a row it broke down into running battles in the middle of Warsaw between rioters and riot police.
Prime Minister Donald Tusk said what should have been a holiday had been ruined by acts of aggression and violence. "What happened is unacceptable," he said.
Several thousand right-wing protesters began their march peacefully - watched by their own stewards in orange vests and with a police helicopter circling above.
The violence started when a few dozen youths, their faces covered by balaclavas and football scarves, broke off from the procession into a side street and started attacking a building where left-wing radicals occupied a squat.
Riot police moved in, and came under attack from youths throwing firecrackers and stones. As the rioters dispersed, several cars were set on fire.
The violence underscores the faultlines in Polish society. Many Poles have grown wealthier in the past few years, but a minority feel alienated and believe traditional values on marriage, abortion and the church have been swept aside.
Their growing profile in Poland mirrors the rise of the far-right elsewhere in Europe. The pain of the economic slowdown around the continent, coupled with local factors, has boosted support for nationalists and anti-immigrant groups in countries from Greece to Hungary and France.
In Poland, non-governmental watchdogs say incidents of racially-motivated violence are increasing, while youths shouting far-right slogans have gate-crashed university lecture halls where liberal academics have been speaking.
Before the violence broke out at Monday's march, demonstrators chanted: "God, honor, fatherland!" and waved the red-and-white national flag.
"I believe that Polishness is under threat," said one marcher, Grzegorz, who said he was 30. "The Polish government is incompetent and a threat to Polishness."
Another protester, who gave his name as Mateusz, said he had come to "manifest my patriotism."
Police spokesman Mariusz Sokolowski said officers had used rubber bullets, truncheons and pepper spray against the rioters. Several people were arrested. Five officers were taken to hospital, local media reported.
When the march reached the Russian embassy - symbol for some Poles of repression during Soviet rule - a part of the crowd threw firecrackers towards the building, but riot police formed a cordon around the perimeter, and the crowd later moved on.
"There is no excuse for hooliganism. We condemn the violation of the Vienna Convention," Marcin Wojciechowski, a spokesman for the Polish Foreign Ministry, said on Twitter in reference to rules on the protection of foreign missions.
The main target of the rioters appeared to have been any symbol of left-wing, liberal views.
After the crowd had moved on from the squat, now littered with burned debris and broken glass, one of the squatters accused the police of failing to hold back the far-right protesters. "You have unleashed fascist dogs on us," he said.
Later, the rioters headed to Zbawiciela Square, one of the most bohemian areas of central Warsaw, where students usually hang out in trendy pavement cafes.
An arch across the middle of the square, decorated in rainbow colors with artificial flowers, had become a symbol of tolerance and diversity. By Monday evening, after the rioters had passed through and set it on fire, all that was left of the arch was a charred, steel skeleton.
(Writing by Christian Lowe; Editing by Janet Lawrence)