WARSAW, Poland (AP) — Police in Warsaw said Monday that four people, including the prime minister's nephew, were detained during a massive protest against government policy that critics say could cost Poland its European Union membership.

Organizers and city authorities say up to 100,000 people took part in the protest in Warsaw on Sunday to show their support for the EU. A nephew of Prime Minister Mateusz Morawiecki alleged that a police officer kicked him in the head while he was on the ground being detained.

Warsaw Police spokesman Sylwester Marczak confirmed the temporary detention of Franek Broda, with the use of handcuffs, but did not address allegations of police brutality. Broda, 18, is a government critic and a LGBT rights activist.

A few dozen people were fined for lighting flares and obstructing traffic during the protest and subsequent march to the headquarters of Poland's ruling right-wing nationalist Law and Justice party.

Similar protests were held in many other cities in Poland.

The protests were sparked by a top court’s ruling last week that the Polish Constitution overrides EU law when they clash. The case was initiated by Morawiecki, who voiced doubts as to EU law's supremacy.

Critics say the ruling by the constitutional court, where many judges are government loyalists, can be seen as a rejection of EU values and may potentially lead to “Polexit,” or the nation being forced out of the 27-member bloc.

Poland's government has been in conflict with the EU for six years as it seeks control over the country's courts and judges. The EU views the pursued changes as an erosion of democratic checks and balances.

EU membership is appreciated in Poland, having brought wide freedoms, including the freedom to travel, and economically transformed the central European nation, which had endured decades of communist rule until 1989.

Morawiecki and party leader Jaroslaw Kaczynski have denied taking a path toward leaving the EU, but the government is increasingly at odds with the bloc's leading bodies.

Morawiecki asked the constitutional court for a review after the European Court of Justice ruled in March that Poland’s new regulations for appointing Supreme Court justices undermine judicial independence and could violate EU law. It ordered the right-wing government to suspend the regulations, which the government has not done.