The exit poll, conducted by the research firm Ipsos, projected that Law and Justice won 43.6% of the votes.
Poland's conservative governing Law and Justice party won the most votes in Sunday's election in the deeply divided nation and appeared, according to an exit poll, to have secured a comfortable majority in parliament to govern for four more years.
The exit poll, conducted by the research firm Ipsos, projected that Law and Justice won 43.6% of the votes. That would translate into 239 seats, a majority in the 460-seat lower house of parliament.
The poll said a centrist pro-European Union umbrella group, Civic Coalition, would come in second with 27.4%. The biggest party in the coalition is Civic Platform, which governed Poland in 2007-2015.
Coalition leaders cheered and welcomed the result as a spur toward uniting society around common goals.
Other parties projected to surpass the 5% threshold to get into parliament were a left-wing alliance with 11.9%, the conservative agrarian Polish People's Party with 9.6% and a new far-right alliance called Confederation with 6.4%.
The exit poll had a margin of error of plus or minus two percentage points. Final vote results, which are expected by Tuesday, could shift, as they have in past elections.
A prominent journalist, Konrad Piasecki, said that "at the moment it looks like the largest triumph in the history of parliamentary elections" in Poland. But he also cautioned that results varying even slightly from the exit poll could mean big changes to the distribution of seats in parliament.
Law and Justice has governed Poland since 2015 and is popular for its social conservatism and generous social spending. It ran a campaign that highlighted its social programs and vowed to defend traditional Roman Catholic values.
It has been accused of weakening the rule of law in the young democracy with an overhaul of the judicial system that has given the party more power over the courts and has drawn criticism as well for using state media as a propaganda outlet.
Pawel Zerka, policy fellow at the European Council on Foreign Relations think tank, said the high level of support for Law and Justice, known in Poland by its acronym PIS, "should not be interpreted as a sign that Poles have become nationalist or xenophobic. Rather, it reveals an effective party machine - and an ability of PIS to mobilise voters with policies based on direct social transfers."
'We received a lot but we deserve more'
Party leader Jaroslaw Kaczynski, who is considered the real power behind Prime Minister Mateusz Morawiecki's government, cautioned that the exit polls weren't the final results but nonetheless declared victory.
"We received a lot but we deserve more," Kaczynski told party supporters as he held high a bouquet of roses.
Civic Platform leader Grzegorz Schetyna said the fight wasn't fair, an apparent reference to the way Law and Justice harnessed state media to pump out positive coverage of itself while casting a poor light on political rivals.
"This was not an even struggle; there were no rules in this struggle," Schetyna said.
The left-wing party leaders celebrated their expected return to parliament after failing to get any seats in 2015.
Critics fear that four more years for Law and Justice will reverse the democratic achievements of this Central European nation, citing the changes to the judiciary and the way the party has marginalised minorities, for instance with its recent campaign depicting the LGBT rights movement as a threat.
Law and Justice's apparent success stems from tapping into the values of the largely conservative society while also evening out extreme economic inequalities.
It is the first party since the fall of communism to break with the austerity of previous governments, whose free-market policies transformed Poland into one of Europe's most dynamic economies.
However, many Poles were left out in that transformation and inequalities grew, creating grievances. Law and Justice skilfully addressed those concerns with popular programmes, including one that gives families a monthly stipend of $125 (500 zlotys) for each child, taking the edge off poverty for some and giving others more disposable income. It says it has been able to pay for its programs thanks to a tighter tax collection system.
It has also clearly benefited from the sacrifices forced by earlier governments and the growth of Europe's economy.
In his victory speech, Kaczynski referred to his party's improvement of public finances and said it would continue on that path.
"We are finishing a certain stage; we are starting a new one," he said. "It is not easier, maybe more difficult. But I hope that it will be finished with even greater success."