Those gathered, who included government critics and leaders of some opposition parties, argued the changes would limit judicial independence and threaten the separation of powers in the country.
Supporters of the governing Law and Justice (PiS) party have criticised Polish courts for taking too long to hear cases, and have accused judges of being an elite, self-serving clique often out of touch with the problems of ordinary citizens.
Demonstrators on Sunday called on President Andrzej Duda to veto the planned changes to the judiciary, saying they were unconstitutional and could harm democracy.
According to a spokesman for Warsaw police, Sylwester Marczak, Sunday's rally attracted up to 4,500 participants.
Grzegorz Schetyna, leader of the largest opposition party, the Civic Platform (PO), and key figures from other opposition groups suggested they were ready to join forces against the country’s ruling conservatives and make efforts to form a single parliamentary caucus.
Economist Leszek Balcerowicz, the architect of Poland’s market reforms after the fall of communism in 1989, said the Law and Justice party was leading the country toward a “socialist dictatorship” in which the judiciary would be controlled by politicians and the courts would lose their independence.
According to government officials, reforms are needed because the country’s judicial system is inefficient and does not serve the interests of the public.
PiS leader Jarosław Kaczyński told reporters on Friday that the Polish judiciary “suffers from two serious illnesses: the first is the collapse of moral principles, professional morality, general morality.
"The second issue is huge inefficiency, delays in cases, which cause many people to suffer in different ways.”
A PiS-backed bill on the National Council of the Judiciary (KRS), a constitutional body tasked with safeguarding the independence of courts and judges, would see the terms of 15 of its members who are judges phased out, and their replacements selected by parliament -- not by other legal professionals, as is currently the case.
A second bill would change the way that heads of district and appeals courts are appointed, making the justice minister solely responsible for such decisions.
Meanwhile, other planned changes would introduce new rules for appointing Supreme Court judges. Current judges of the court could be retired under a PiS-backed bill.
As the crowd at Sunday’s protest chanted slogans against those in power and “in defence of the courts," a group of government supporters held banners applauding Kaczyński and Prime Minister Beata Szydło for working to make “Poland great” again. (str/pk)