Thousands joined march in support of liberal opposition in Warsaw and other cities

Crowds stretching for at least a mile marched with banners reading "Free, European Poland", "European Union yes, PiS no".

Some held masks of ruling party leader Jarosław Kaczyński that had the word "shame" written on them. Half a million people were marching, according to organizers. Police and city officials did not give an estimate.

"I'm here because freedom is important, because everyone should feel safe, this is a protest of free people, I believe that change is already happening," Ewa Joachimowicz, a marcher from Chełm in eastern Poland, told Reuters.

Opinion polls show an election due after the summer will be closely fought, with Russia's war in neighbouring Ukraine giving a boost to the Law and Justice (PiS) government which has emerged as a leading voice against the Kremlin in Europe.

The opposition has struggled to galvanise support despite widespread criticism at home and abroad of the PiS, which has been accused of eroding the rule of law, turning state media into a government mouthpiece and endorsing homophobia.

The government of Prime Minister Mateusz Morawiecki denies subverting any democratic norms and says its aim is to protect traditional Christian values against liberal pressures from the West and to make the economy more fair.

Donald Tusk, head of the Civic Platform grouping and former European Union council chief, welcomed supporters saying that the voice of Poles cannot be silenced. "The first step to victory is to recognise our strength, we're here so that Poland, Europe and the world see our strength," Tusk told the crowds.

"This wave will not be stopped, that giant has woken up, I'm proud that I can be here and say we will win!"

In 1989, the partially free vote on June 4 handed victory to a government led by the Solidarity trade union and triggered a series of events culminating in the fall of the Berlin Wall that November.

On Sunday, hundreds of buses were arriving in Warsaw to bring opposition supporters from across the country. Some said they were motivated by a row over legislation proposed by PiS to weed out undue Russian influence from the country.

The opposition sees the legislation as a government attempt to launch a witchhunt against political opponents.

In an unexpected turnaround, President Andrzej Duda, a PiS ally, said on Friday he would propose amendments to the law, which has also drawn criticism from lawyers, as well as the U.S. State Department and European Commission.

The EU's executive said the legislation could effectively ban individuals from holding public office without proper judicial review.

"It's beyond comprehension," said Andrzej Majewski, 48, from Słupca in western Poland who was in Warsaw to join Sunday's protest march.