Thousands of opposition supporters have taken to the streets of Poland's capital Warsaw in the latest round of a constitutional row that has put the government on a collision course with the country's top court.

After sweeping to power last October, Poland's conservative Law and Justice (PiS) party enacted a law increasing the number of judges at the constitutional court required to make rulings and changing the order in which cases are heard.

The court, however, has said the new rules are illegal, effectively putting the changes in limbo.

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An estimated 50,000 anti-government supporters rallied on Saturday in front of the court and then marched across Warsaw to the presidential palace.

"Poles used to demonstrate to change the state. Today they demonstrate to preserve it," Ryszard Petru, head of the liberal-conservative party Nowoczesna, told the crowd of protesters.

The demonstrations, also held in the cities of Poznan and Wroclaw, came as the government refused to publish a ruling by the constitutional court abolishing the new laws.

"We uphold the position that Poland's government cannot publish the statement of some of the constitutional court judges, which is not based on law," government spokesman Rafal Bochenek told reporters on Saturday.

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On Friday, experts from Europe's leading human rights watchdog said the legal reforms "would undermine democracy, human rights and the rule of law".

But the government has argued the constitutional court is too powerful, allied to the last administration and determined to block reforms the party was elected to push through - charges dismissed by the court and rights groups.

The crisis, which has caused concern in the European Union and the United States, appears to be the most serious since Poland threw off communism in 1989.

The government, however, denies that democracy is threatened.

"Democracy is fine, very fine," Beata Kempa, a leading official in the government of Prime Minister Beata Szydlo, said recently.

"We don't send police with bullets against people. They are allowed to express their views here."