An estimated 500,000 people marched through downtown Warsaw Sunday afternoon in a huge rally against the ruling Law and Justice (PiS) party, held on the 34th anniversary of the breakthrough election that effectively ended communist rule in Poland.

The march was called by Donald Tusk, former Polish prime minister and former president of the European Council, who is now leading his Civic Platform (PO) party in a bid to defeat PiS in an election due later this year.

“You’re all here because you have just believed we can win,” Tusk told the crowd, which filled Warsaw’s Castle Square. People in the crowd were holding up white-and-red flags and anti-government placards, including the locally famous ones with eight asterisks, denoting the Polish for “f*** PiS.”

“Here’s my pledge to you today: We are going to win this election and hold PiS accountable,” Tusk said.

The rally, which appeared to be the biggest political demonstration in Poland in decades, took place just a few days after PiS-friendly President Andrzej Duda signed off on controversial legislation setting up a special commission to probe Russian influence on Poland’s security. The commission will have vast powers including slapping whoever is found to be making political decisions under Russia’s sway with a 10-year ban from holding public office.

Poland’s opposition has said that the proposed sanctions of the special commission could be used to remove Tusk from politics under trumped-up allegations.

Following criticism in Poland, as well as from the U.S. and the EU, Duda has since moved to blunt the commission’s powers. That would, however, require another vote in the parliament.

Poland is gearing up for what is set to be a close election this fall. According to POLITICO’s Poll of Polls, Tusk’s Civic Platform is currently trailing PiS. But the ruling party does not have enough support to guarantee it a majority in the next parliament.

The standing of PiS in the polls has weakened in 2023 in the wake of double-digit inflation and an economic slowdown. Both are expected to ease later this year, possibly giving the ruling party some room to outdo the opposition and secure an unprecedented third consecutive term in office.