Half a million people gathered in the centre of the Polish capital Warsaw to protest against the ruling nationalist conservative Law and Justice (PiS) party at the call of former Prime Minister Donald Tusk.

Demonstrators held placards with slogans such as "Europe, we apologize for PiS" and "PiS in the urinal."

The call by Tusk, leader of the liberal Civic Platform party, was supported by other opposition formations. The protest was joined by the winner of the Nobel Peace Prize for 1983 Lech Walesa, co-founder of the historic trade union "Solidarity" and former president of the country.

The demonstration marked the first partially free elections in then communist Poland. Produced on June 4, 1989, they were seen as a victory for the pro-democracy movement and Solidarity, the then Soviet bloc's first independent trade union, and ended communist rule in the country.

"We are here so that all of Poland, all of Europe, the whole world can see how strong we are, how many of us are ready to fight for freedom and democracy, as we did 30 and 40 years ago," Tusk told the crowd.

The next regular parliamentary elections in Poland are due to be held in October or November this year.

Poland is providing farmers with 2.4 billion euros in aid for the construction of a new gas pipeline. Dollars