Polish riot police used batons, teargas and water cannon against thousands of right-wing demonstrators after Independence Day observances Friday turned into clashes in which police were attacked by leftist and right-wing marchers, witnesses said.

The police, attempting to keep the nationalist and leftist demonstrators apart, responded after right-wing marchers began pelting them with stones, bottles and flares, police spokesman Mariusz Sokolowski said.

More than a dozen people, including nine policemen, were injured in the clashes, and rightist demonstrators later set fire to a television van covering the unrest.

In an earlier development, leftist demonstrators chanting Fascism will not pass attacked police trying to stop them blocking Marszalkowska Street, a major Warsaw thoroughfare, where a nationalist march was due to pass, Sokolowski said.

Prime Minister Donald Tusk has ordered all public services to maintain strict order and severely penalise all law breakers, the Government Information Centre said in a statement following the disturbances during which some 150 people were detained.

The police spokesman said demonstrators identified as having attacked the police would be prosecuted. The identification process is continuing, he added.

Under Polish law, an assault on a law-enforcement officer carries a maximum three-year jail sentence.

For weeks, a coalition of leftists, anarchists, pro-abortionists, Greens and gay-rights activists had been publicising plans to block the Independence March being organised by nationalist youth groups All-Polish Youth and the National Radical Camp.

The November 11 celebration marks the day in 1918 when Poland regained its independence after having been carved up for 123 years by Russia, Prussia and Austria.

Polish President Bronislaw Komorowski said late Friday he already has asked lawyers to check whether it's possible to change a law regarding assemblies in order to avoid such incidents in the future.

It is in the interest of organisers (of such events) to be clear who is a perpetrator and may be a source of a shame because of such dishonourable behaviour, the president said.

Speaking at an official ceremony earlier, Komorowski had urged Poles to celebrate this patriotic occasion together, not against one another.

(Reporting by Rob Strybel and Dagmara Leszkowicz; Editing by Michael Roddy)