Sunday’s ballot was seen as an important test of support both for opposition groupings which - unusually - united behind one candidate, and for the Law and Justice (PiS) party, which has governed Poland since late 2015.
An exit poll found that opposition candidate Konrad Fijołek won 55.8 percent of the vote, while the contender backed by PiS, Ewa Leniart, garnered 25.1 percent.
The survey by pollster IBS found that the two other candidates running, both right wingers, trailed far behind, with support at less than 10 percent.
Fijołek, as cited by radio broadcaster RMF, said the results were a victory for unity, adding: “From here, from Rzeszów, a return to … freedom and democracy will begin.”
The Polish opposition, which has been riven by internal feuding, accuses the governing Law and Justice party of undermining democratic norms and the rule of law during its five-and-a-half years in power, charges which the government denies.
The official results of the election are expected on Monday.