Opposition leader Donald Tusk told supporters that political “change for the better is inevitable” in Poland as he opened a march on Sunday.

The rally was held to try to boost the chances of the political coalition built by Mr Tusk of unseating the country’s conservative government in an upcoming parliamentary election.

“No-one can stop this force; this giant has awoken,” he told huge crowds gathered in the centre of Warsaw two weeks before the election on October 15. “Let no-one among the ruling team have any illusions: This change for the better is inevitable.”

His Civic Coalition is vowing to mend ties with the European Union, which has had strained relations with Poland during the eight years the Law and Justice party has governed the country. Mr Tusk spent five years as president of the European Council after serving seven years as Poland’s prime minister.

The four-party coalition has also pledged to pursue more tolerant policies than the nationalist government led by Law and Justice.

Thousands of supporters arrived on buses provided for them from across Poland to take part in the “March of a Million Hearts”. Police closed some streets for the 2.5-mile walk. People in the crowd carried national and EU flags.

“When I see this sea of hearts, I can sense that a breakthrough moment is coming in the history our our homeland,” Mr Tusk said to cheers and chants of his name.

Marchers interviewed by Polish private news channel TVN24 said they were taking part in the interest of their children, grandchildren, women and LGBTQ+ people whom they want to live in a modern, tolerant and European Poland. Participants sang the national anthem at the end of the event.

Civic Coalition rallies were also held in some other Polish cities. A centrist opposition alliance, the Third Way, abstained from the march in the capital and held its own rallies. Law and Justice was holding a party convention in southern Poland’s city of Katowice.

Mr Tusk’s electoral alliance is a few percentage points behind Law and Justice in recent polls. He says the wider opposition that includes the Left party and Third Way could defeat the ruling party and form a government. He greeted Third Way’s leaders at the start of his march.

Warsaw city government spokesperson Jakub Leduchowski estimated the crowd size at about one million people, and Mr Tusk called the march the “world’s biggest political gathering” of the day. Organisers estimated a similar Civic Coalition march in June drew about 500,000 participants.

Mr Tusk, 66, returned to Polish politics several years ago, seeking to breathe new life into his languishing Civic Platform party and to reverse what many view as a degradation of fundamental rights under the Law and Justice-led government.

The governing party and the government itself have waged a hostile and aggressive campaign. The hard-right Confederation party has also been growing in popularity.