Poland's main opposition party on Friday chose Warsaw's mayor as its new presidential candidate, just hours after its previous choice withdrew from a chaotic race delayed by the coronavirus pandemic.

The liberal Civic Platform (PO) picked Rafal Trzaskowski, 48, to replace Malgorzata Kidawa-Blonska who stepped down earlier in the day.

Trzaskowski, a former Europe minister and member of the European Parliament, won a crushing first-round mayoral victory in Warsaw in 2018 against the candidate of Poland's governing right-wing Law and Justice (PiS) party.

He will now lock horns with incumbent President Andrzej Duda, the election frontrunner who is backed by the PiS.

"I'm assuming the huge responsibility of fighting for a strong state and for democracy," Trzaskowski told reporters, referring to controversial PiS judicial reforms that have raised concern about the rule of law in Poland.

Trzaskowski beat MEP Radoslaw Sikorski, a former foreign minister, in the race to challenge Duda.

While the PiS ally has long been the election favourite, recent polls suggest Duda can no longer count on a first-round victory as the pandemic-induced economic crisis bites.

Originally set for May 10, the election was postponed at the last minute when the government and opposition parties failed to agree on how to proceed under the coronavirus lockdown.

A new election date has not yet been set and parliament is currently struggling to overcome the stalemate over how to hold the election safely and in line with the constitution.

The PiS wants the ballot to go ahead on June 28 or in early July before Duda's five-year term ends on August 6th.

Poland's opposition Civic Platform (PO) picked Warsaw mayor Rafal Trzaskowski, 48, as its new presidential candidate just hours after its previous candidate,  Malgorzata Kidawa-Blonska withdrew
Poland's opposition Civic Platform (PO) picked Warsaw mayor Rafal Trzaskowski, 48, as its new presidential candidate just hours after its previous candidate, Malgorzata Kidawa-Blonska withdrew AFP / Attila KISBENEDEK

Opposition parties rejected earlier PiS government plans for a universal postal ballot, insisting the constitution requires any change to voting rules to be made at least six months before election day.

They also insist that the way PiS party leader Jaroslaw Kaczynski postponed the May 10 election date, via a political declaration, was illegal.

Constitutional experts say a government can only postpone an election by declaring a state of emergency or disaster, something the PiS administration has refused to do despite the unprecedented social and economic impact of the pandemic.

Kidawa-Blonska quit Friday after her popularity sank to the bottom of the rankings.

She had suspended her campaign and called for a boycott of the original May 10 election date, insisting that lives would be at risk should the ballot go ahead during the coronavirus pandemic.

"I'm responsible for my ratings falling because Poles didn't know whether or not I would take part in the election, but for me the most important was voters' health, safety and that Poland would remain a democracy," she said in her resignation speech.

The crisis comes within the broader context of long-standing EU concerns about democratic standards in Poland.

The European Commission has launched four infringement procedures against PiS-authored judicial reforms, which it says test democracy and the rule of law by undermining judicial independence.