WARSAW, Poland (AP) — Polish opposition leader Donald Tusk is leading a march Sunday in Warsaw to mobilize supporters and boost the chances of unseating the conservative government in upcoming parliamentary elections.

Thousands of supporters were arriving on buses provided for them from across Poland to take part in the “March of a Million Hearts.” Police closed some downtown streets for the 4-kilometer (2.5-mile) walk.

Rallies are also to be held in other cities.

Tusk has been facing a hostile and aggressive campaign on the government’s side and his electoral alliance, the Civic Coalition, is trailing in surveys a few percentage points behind the ruling Law and Justice party. Still, Tusks says there are chances for the wider opposition that includes the Left party and centrist Third Way to win in the Oct. 15 vote.

A similar march in June drew huge crowds that the organizers estimated at some 500,000.

Tusk, 66, an ex-prime minister and former European Union leader, returned to Polish politics several years ago. He is seeking to win back power and reverse what many view as a degradation of fundamental rights and ties with European partners under the eight years of Law and Justice rule.