Poland's liberal opposition parties on Friday signed a coalition deal they said would be a "roadmap" for the new alliance if it was given the chance to take power.
"We really wanted Polish women and men who went to vote with such hope to see that... from now on, we are ready to take responsibility for our homeland and for the coming years," Donald Tusk, the opposition's candidate for prime minister, told reporters.
Tusk, a leader of the liberal Civic Coalition main opposition bloc, spoke alongside his counterparts from the centrist Third Way and the Left.
"When I look at you all, you sort of remind me of the Avengers with superpowers," said Left co-head Robert Biedron, referring to the comic book team of superheroes.
While the governing conservatives received the most votes in last month's election, they fell short of a parliamentary majority.
Civic Coalition finished second but the opposition alliance as a whole secured a majority of 248 lawmakers in the 460-seat parliament.
The leaders of alliance had jointly called on Polish President Andrzej Duda to nominate Tusk for prime minister.
The head of state however gave the governing Law and Justice (PiS) party -- his allies -- a first shot at forming the new government.
On Monday, Duda tasked current Prime Minister Mateusz Morawiecki with the mission.
It is unclear who the PiS, which obtained 194 seats, could govern with as observers note the lack of viable coalition partners.
Tusk, who is a former prime minister and ex-head of the European Council, has accused Duda of "playing for time" in selecting Morawiecki.
Tusk said Monday that the move "will not change anything", suggesting that he himself would ultimately become the head of government.