The spokesman for Poland's president says the leader sees flaws in the contentious legislation adopted by the Senate that would give politicians significant influence over the country's top court.
On Wednesday, the European Union gave Poland a week to shelve the judicial reforms that Brussels says would put courts under direct government control.
The bill is sponsored by the populist ruling Law and Justice party. The head of state has 21 days to sign the document, veto it, or, if in doubt, submit it to the constitutional court.
If the new regulations come into law, they will force all of the Supreme Court's judges into retirement and give the president powers to choose who to reinstate. Opponents say the legislation will destroy judicial independence and violate the rule of law.
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The European Commission urged the Polish authorities to refrain from changes in the judicial system.
If the PiS government does not back down, Poland could face fines and even a suspension of its voting rights, although other eurosceptic European Union governments, notably Hungary, are likely to veto strict punishments.
But the governing Law and Justice (PiS) party has said sweeping changes are needed to reform an inefficient and sometimes corrupt judicial system. "We will not be intimidated by Polish and foreign defenders of the interests of the elite", Prime Minister Beata Szydlo said in an address on state television. Critics concede that the judiciary needs reform, but that the proposed changes are going in the wrong direction.
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He said: "I'm calling on the opposition to return to peace, to respect for the constitution, to sit and talk". That raised the possibility of a parallel state structure to challenge government-controlled institutions.
From a window high up in the building, a former leader of the Senate, Bogdan Borusewicz, called out encouragement to protesters, saying, "You are giving us strength".
The KRS revamp has been approved by parliament and is awaiting Polish President Andrzej Duda's signature. The bill passed 235-192 with 23 abstentions. Marek Pek, a Law and Justice senator, said the aim is to restore the proper balance of powers.
The second bill was approved by the lower chamber of Parliament Thursday-triggering the pro-democracy protests-but still must be approved by the upper house, then signed by Duda, to take effect.
Tusk, former Polish prime minister, wrote in a statement Thursday that "Subjecting the court to one ruling party in the way that Law and Justice has proposed it will ruin already strained opinion on Poland's democracy".
In Poland, past presidents, activists and rights groups were among those to speak out against the judicial reforms, raising concerns about the erosion of laws and freedoms in Poland.
The threat to the independence of the Supreme Court is the latest maneuver by the right-wing party, which gained power in 2015, to bring the judiciary under its control.
While PiS remains broadly popular among many Poles, particularly poorer and older voters from the countryside, there have been widespread protests against the plans.