Thousands of Poles took into streets in support of European Union membership on Sunday.
The rallies took place in more than 100 Polish towns and cities, and several cities abroad.
This came after a controversial court ruling raised concerns the country could eventually leave the EU membership.
The Poland’s Constitutional Tribunal ruled on Thursday that parts of EU law are incompatible with the country’s constitution.
The court order may undermine the legal pillar on which the 27-nation bloc stands.
The European Commission (EC) president, Ursula von der Leyen, said she is extremely concerned by Poland’s ruling.
Also, she said the EC will do its best to make sure the EU’s primacy is of priority.
The largest of the demonstrations was in the capital, Warsaw. Some waved Polish and EU flags while crowds shouted “We are staying”.
However, police forces detained four people during the protest, including Polish Prime Minister Mateusz Morawiecki ‘s nephew.
Franek Broda, Morawiecki’s nephew, alleged that a police officer kicked him in the head while he was on the ground before his detention.
Warsaw Police spokesman Sylwester Marczak confirmed the 18-year-old had been temporarily detained with the use of handcuffs, but did not address allegations of police brutality.