The Law and Justice (PiS) party won 43.6 percent of the vote, according to the survey by pollster Ipsos, a result that would give it 239 seats in the 460-seat lower house.
State broadcaster TVP reported that was the best Polish parliamentary election result by any party in the last 30 years.
Parties from across the political spectrum have said the election is the most important since the collapse of communism in 1989 and that it would decide the future direction of country riven by bitter political divisions between conservatives and liberals.
Turnout was a record 61.1 percent, according to the exit poll. The official election results were expected to be announced by Tuesday.
Law and Justice leader Jarosław Kaczyński claimed victory, telling cheering supporters on Sunday: “We’ve won; we’ve managed to win despite a powerful front against us.”
He added: “We have four years of hard work ahead of us because Poland must continue changing, and it must be changing for the better.”
'Enormous social mandate'
Prime Minister Mateusz Morawiecki said the exit polls results showed that Law and Justice had been given an “enormous social mandate.”
The largest opposition bloc, the Civic Coalition, led by the Civic Platform (PO) party, was runner-up in Sunday’s election, with 27.4 percent support, according to the exit poll. State broadcaster TVP predicted that would give the party 130 seats in the lower house.
Civic Platform leader Grzegorz Schetyna said after exit poll results came through: “We didn't feel that we were taking part in a fair battle, that our opponent was using fair methods.”
Left-wingers back in
A block of three leftist parties led by the post-communist Democratic Left Alliance (SLD) finished third on Sunday with 11.9 percent, according to the exit poll, a result that would translate into 43 seats and open the way for a left-wing grouping to return to parliament after a hiatus of four years.
The Polish Coalition of the rural-based Polish People’s Party (PSL) and the anti-establishment Kukiz’15 grouping finished fourth with 9.6 percent, which would give the grouping 34 seats, the poll found.
The right-wing nationalist Confederation group, with 6.4 percent, also crossed the 5-percent voter support threshold that Polish parties need to clear to enter parliament. That result would give it 13 seats, according to the exit poll.
Triumphant Law and Justice supporters claimed the result vindicated the party’s determination to share the benefits of the country’s booming economy with poorer citizens, while defending traditional Catholic values, fending off what it sees as Western political correctness, and seeking to ensure Poland punches its weight on the international arena.
With an opposition divided into separate blocs, pre-election polls had indicated that Law and Justice was well ahead, buoyed by an economy that has been one of the strongest performers in the EU and by historically low unemployment.
Economy doing well
Polish GDP grew 5.1 percent in 2018, the third-fastest economic growth that year in the European Union, which Poland joined in 2004.
Since coming to power in a landslide 2015 election win, Poland’s conservatives have introduced popular benefits for families with children, increased the minimum wage and promised extra handouts for pensioners, while securing extra revenues through a high-profile clampdown on VAT fraud.
Despite criticism that the conservatives would not be able to afford their swathe of spending pledges, Poland's government last month approved a balanced budget for 2020 that expects the economy to grow 3.7 percent, with inflation targeted at 2.5 percent
Opposition parties, led by the economically liberal Civic Platform, which was in power from 2007 to late 2015, claim that Law and Justice has eroded democracy and judicial independence, especially through sweeping changes to the justice system which have brought Warsaw into conflict with Brussels.
Law and Justice vehemently denies the charges.
The election results were expected to be formally announced on Tuesday, though officials have not ruled out that they may be known sooner.