Demonstrators chanted that the ruling party’s leader is “introducing dictatorship” and “dismantling” the rule of law.
But the government has said the changes are needed to reform an inefficient and sometimes corrupt judicial system.
Supporters of the ruling, conservative 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.
The protest at the Supreme Court followed a demonstration outside the Polish parliament in Warsaw and in several other cities across the country earlier on Sunday.
A spokesperson for the National Council of the Judiciary (KRS), one of the bodies set to be reformed, said: “We are at the precipice of dictatorship and breaking all the rules of the constitution”.
The protests throughout the country were against a number of planned changes to Poland’s legal system, including to the Supreme Court.
A government-backed bill would see the court’s existing judges retired and reinstatement only possibly on the justice minister’s approval.
The opposition said this would give huge power to the justice minister and would undermine the independence of the court.
But the government said that, under the proposed law, only “highly qualified” and “ethical” people could become judges.
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.
Yet another bill would change the KRS, a constitutional body tasked with safeguarding the independence of courts and judges.
It 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.
Justice Minister Zbigniew Ziobro told public broadcaster TVP that the changes serve democracy and citizens.
He added that the justice system would be equipped with “tools to deal with increasingly frequent irregularities in the judiciary”.
He claimed the Polish judiciary had “almost totally broken away from democratic control mechanisms”.
Poland is already embroiled in a row with Brussels over PiS's sweeping changes to the Constitutional Tribunal and concerns about the rule of law, which have caused bitter divisions in Poland and concern abroad. (vb/pk)
Source: IAR, PAP