The election will be "a coup d'etat not only against our democracy but also against the life and health of Poles," opposition Civic Platform presidential candidate Malgorzata Kidawa-Blonska told Poland's Fakt newspaper in a video posted on Saturday.
"This election will not be democratic and the result is fixed in advance. We cannot take part or encourage Poles to take part in this farce," she said. "This election must not go ahead on May 10."
Civic Platform's president Borys Budka said "the health and life of Poles" are at stake whether the election takes place, reported The Associated Press. "No responsible person would call for participation in such a vote."
Civic Platform, Poland's main opposition party, is calling for the vote to be postponed by a year.
Reducing the risk
PiS says the date of the election is dictated by the country's constitution and that going ahead with the mail-in voting would be a democratic victory that poses no health risks to those voting.
Health Minister Lukasz Szumowski commented Friday to Polsat television that a safe physical ballot could only take place once there was a vaccine for COVID-19.
Under the rules for the upcoming election, there will be no physical polling stations. Instead, all ballots will be delivered by post. Each person will drop their vote in special post boxes in their local area, which will be sent for counting.
There is uncertainty around how well government agencies will be able to organize a postal vote for around 30 million people for the first time, with just over a month to prepare.
EU Values and Transparency Commissioner Vera Jourova told Polish daily Rzeczpospolita earlier this month that she was "concerned about free and fair elections and the quality of voting, of the legality and constitutionality of such a vote."
EU lawmakers recently called the Polish government's postal vote "totally incompatible with European values."