Former EU chief Donald Tusk returned to frontline politics against Poland's governing right-wing populists on Saturday, taking over as acting head of the opposition Civic Platform party.
The 64-year-old co-founded Civic Platform (PO) two decades ago and served as prime minister of Poland between 2007 and 2014.
"I am back 100 percent," Tusk told a party conference, where he received a standing ovation.
He spoke of the "evil" of the ruling Law and Justice (PiS) party and accused it of putting Poland in a "dangerous situation" by falling out with EU partners.
"When you see evil, fight it," he said.
In a vote by the PO's national council, Tusk was elected party vice-chair which means he will automatically be acting head until a leadership election expected later this year.
Tusk is currently chairman of the centre-right European People's Party and has been a frequent acerbic commentator on social media from the sidelines of Polish politics.
A self-avowed football hooligan in his youth, Tusk grew up in the northern port city of Gdansk which was the cradle of the Solidarity movement that dismantled Communist rule.
As president of the European Council from 2014-2019 he handled crises ranging from migration to Greece's economic plight and tough Brexit negotiations.
But commentators said he will face a major challenge uniting fractious opposition forces.
Maciej Gdula, an MP from the Left party, criticised Tusk's speech for "a lot of emotional criticism of PiS, little about the future".
Tusk will also have to find a way to work together with Rafal Trzaskowski, the mayor of Warsaw and runner-up in the presidential election last year, who is also believed to have leadership ambitions.
But MPs at the party conference were optimistic.
"Civic Platform will be great again," party MP Robert Tyszkiewicz tweeted after Tusk's announcement.
- Poland in 'complete isolation' -
PO has lagged third in the polls behind PiS and the opposition Poland 2050 party.
At the same time, the PiS has lost its narrow majority in parliament and relations with the EU are plunging on a number of issues, including controversial judicial reforms.
Tusk said PiS leader Jaroslaw Kaczynski had led Poland into "complete isolation" on the international stage.
Elections in Poland are currently scheduled for 2023 but commentators say the governing party could be tempted by early elections to avoid a no-confidence vote in parliament.
The PiS also held its own leadership congress on Saturday at which the party confirmed 72-year-old powerbroker Kaczynski as leader.
Kaczynski told delegates he would be leader of PiS "for one final time" and promised to press ahead with the party's economic programme to bring living standards in Poland in line with Western Europe.
A new poll this week showed the governing right-wing coalition dominated by the PiS in the lead with 34 percent, followed by Poland 2050 on 17.1 percent and PO on 16.9 percent.
The survey was carried out by the IBRiS polling company for TV channel Polsat.