The exit poll showed four other parties clearing the hurdle to enter parliament, with the centrist Civic Coalition on 27.4 per cent, the left-wing Lewica alliance on 11.9 per cent, the PSL-Kukiz'15 alliance of agrarians and populists on 9.6 per cent and the far-right Confederation with 6.4 per cent. The conservative agrarian Polish People's Party got almost 9%, while Confederation, a new far-right group that is openly anti-Semitic and homophobic, is set to enter parliament after winning 6.8% of the vote.
More than 30 million voters were choosing lawmakers in the 460-seat lower house of parliament, the Sejm, and in the 100-seat Senate.
Turnout, at 61.6 per cent, was the largest in a Polish parliamentary election since the fall of communism in 1989.
Two separate opinion polls published Friday suggested the PiS's majority is at risk as it scored 40 to 41.7 per cent support compared to a combined 41.4 and 45 per cent for opposition parties.
"We did not have the feeling that we competed in a fair fight, that our opponent used fair methods", Grzegorz Schetyna, the head of the Civic Platform, the KO's main party, said.
Opposition hopes of winning enough seats to form a coalition government even if PiS topped the vote looked in disarray after a bitter campaign in which the ruling populists positioned themselves as defenders of "Polish values".
However, for many voters, these institutional reforms seem to be less important than the broad social benefits introduced by PiS during its term of office, including the flagship 500 zloty ($125) monthly benefit per child, as well as the party's attachment to conservative values.
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A few percentage points difference in official results would affect these calculations, so many in Poland are taking Sunday's figures with a grain of salt.
Condemning the anti-LGBT drive and close church ties, but sharing the PIS's welfare goals, the left is set to return to parliament after a four-year hiatus.
"We have victory", PiS leader Jarosław Kaczyński told supporters at party headquarters in Warsaw.
Law and Justice's overhaul of the judicial system has given the party unprecedented power over the public prosecution system and the courts.
However, many Poles were left out in that transformation and inequalities grew, creating grievances that Law and Justice has addressed skillfully.
Party leader Jaroslaw Kaczynski has declared victory and said that the positive changes will continue. The EU has repeatedly warned that the rule of law is threatened and has sanctioned the country, preventing some of the changes, but not all.