Thousands of far-right sympathisers are expected to march through Warsaw on Thursday in an annual Independence Day gathering after Poland's nationalist rulers helped challenge a court ban on the event.

Critics say in lending a hand to the Nov. 11 march, an event marked by occasional violence, the Law and Justice (PiS) government is giving overt support to the far-right, as Poland faces an unprecedented migration pressure along its eastern border.

It has also become a point of friction between the liberal opposition on one side and the PiS government and far-right organisers on the other, as critics accuse PiS of fomenting anti-migrant sentiment and homophobia.

"PiS has... taken responsibility for whatever happens during the march, every fight, every instance of arson," the Left opposition group said on Twitter.

A PiS spokeswoman declined to comment on whether PiS is endorsing the march and directed questions to a veterans' agency that made the formal announcement.

Organizers vowed the march would be held in honour of uniformed officers acting as "protectors of the border", and would highlight the importance of protecting Poland's sovereignty against intruders.

In an escalation of a border crisis that threatens to draw in Russia and NATO, the European Union accused Belarus on Wednesday of mounting a "hybrid attack" by forcing people into Poland, paving the way for new sanctions against Minsk.

Poland also faces its worst conflict with the EU in years over accusations that it is subverting the rule of law.

Warsaw city authorities challenged the registration of the march in court and won both the first instance case and an appeal. The mayor of the capital said that the march would be unlawful if it were to go ahead.

On Tuesday, the head of the Office for War Veterans and Victims of Oppression said he had given the march formal status, therefore allowing it to move ahead, calling the decision of Warsaw's mayor "incomprehensible".

Representatives of other European far-right groups, including the Hungarian Our Homeland Party (Mi Hazank), are expected to take part in the march.

The march comes as Poland is in the middle of a fourth wave of the coronavirus pandemic.