Tens of thousands of people marched in the Polish capital Warsaw, accusing the new government of trying to manipulate state institutions.
The Committee for the Defence of Democracy, which organised the protest, says President Andrzej Duda is breaking the law in his appointment of judges.
Mr Duda is an ally of the Law and Justice party, which won elections in October.
MPs have accused the party of carrying out a "creeping coup d'etat".
The protests are centred on a dispute about the powers of Poland's Constitutional Court, which can block legislation. The government says the court is biased because it is run mainly by judges appointed by the previous government.
The government ignored two of the court's rulings in December.
Around 50,000 people marched through the streets of Warsaw, with some chanting "Duda must go", according to AP.
Others carried banners calling on Jaroslaw Kaczynski - leader of the Law and Justice party - to leave Poland alone.
"Together we will stand as a non-partisan front to protect democracy and show our discontent regarding what is being done to institutions in a democratic state," the founder of the KOD movement, Mateusz Kijowski, told Radio Poland.
Opposition parties, including the Civic Platform and the Modern party, have also criticised the government, according to local media.