Officers came under attack near de Gaulle roundabout in the centre of the capital, Polish media reported.
"In order to restore order, means of direct coercion are being used," police said in a statement.
The onet.pl website reported that a number of people had been injured and police had made arrests.
It added that several thousand people had gathered in central Warsaw, even though the organisers of an annual Independence Day march had said it would take place in motorised form this year.
Police said that a helicopter had been deployed and officers were monitoring trouble via surveillance cameras.
Polish Prime Minister Mateusz Morawiecki last week appealed for the Independence March to be called off amid a surge in COVID-19 infections.
Gatherings of more than five people have been forbidden in Poland under rules aiming to curb the spread of the coronavirus, and the government has warned that a nationwide lockdown may be imposed if cases continue to surge.
In a televised address, President Andrzej Duda on Wednesday told Poles that abiding by government restrictions was a form of patriotism.
Source: IAR/Polish Radio/onet.pl
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