A computer system failure is delaying returns from Poland's local elections while exit polls suggest the nationalist opposition winning in the countryside and the ruling party taking the big cities.

Based on exit polls, Prime Minister Ewa Kopacz has congratulated the opposition Law and Justice party for its apparent victory before the 2015 general election.

The State Electoral Commission said Monday that the computer problem has been fixed, but it wasn't clear when the full returns would be known.

Some cities may need runoffs to choose their mayors. According to an Ipsos exit poll, Law and Justice won about 31.5 percent of the provincial assemblies votes, compared to around 27.3 percent of votes for the ruling Civic Platform.