Prime Minister Jaroslaw Kaczynski has admitted defeat in Poland's general election, after exit polls predicted victory for the Civic Platform party.
Mr Kaczynski said his conservative Law and Justice Party had "failed against a wide front" after it polled about 31%, while its centre-right rival got 44%.
The election was called two years early after Mr Kaczynski's coalition collapsed over a corruption probe.
Turnout seemed substantially higher than the 12-year low in the 2005 poll.
The prime minister's twin brother, Lech Kaczynski, is Poland's president.
The Law and Justice Party (PiS) has pursued former communists and adopted a sceptical approach to the European Union, while Civic Platform (PO) has promised a more business-friendly administration with closer ties to Europe, correspondents say.
Voting in Sunday's election was extended for three hours in some parts of the country as officials struggled to cope with the sharply increased turnout of 55% - the highest since the fall of communism in 1989.
Shortly afterwards, exit polls for the TVP state television network suggested Mr Kaczynski's party had received 30.4% of the vote, some 13% less Donald Tusk's Civic Platform.
Another exit poll by the private TVN24 channel predicted the Civic Platform would win 44.2%, leaving the Law and Justice Party on 31.3%.
If correct, the Civic Platform and its preferred coalition partner, the Polish Peasants Party, will win a majority in the 460-seat lower house of parliament, the Sejm.
After the polls were announced, Mr Kaczynski admitted defeat and congratulated his rival leader at a speech to supporters at his campaign headquarters.
"We have failed against a wide front," he said. "I wish Donald Tusk every success and I congratulate him."
Mr Kaczynski blamed the election result on a coalition of newspapers and political parties, who he said had stopped at nothing in their campaign to oust him.
He said the high turnout was a sign of the strength of Polish democracy and that his party should be proud of the five million votes he said it had won, almost two million more than two years ago.
In conclusion, Mr Kaczynski promised the Law and Justice Party would be a "decisive, tough opposition" in parliament and that future elections would be happier it.
The BBC's Nick Thorpe in Warsaw says the prime minister's speech set the tone for his party - a mix of defiance and despondency.
Only two years after they came to power, one of Europe's most colourful and controversial governments has been voted out of office, our correspondent says.
Earlier, Mr Kaczynski chatted with voters as he queued at a polling station in Warsaw.
"We have to accept the will of the voters, that's obvious," he said after voting, according to Reuters news agency.
"We won't get angry at the people and lessons from this campaign will be learned."
His rival, Mr Tusk, also cast his vote in the capital city.
"Of course I expect to win, but I also know perfectly well that it will not be easy and the battle goes on until the last minute," he was quoted as saying.
Poles have become disillusioned with democracy following a succession of unhappy coalition governments, says the BBC's Adam Easton in Warsaw.
The country is polarised over the figure of the 58-year-old prime minister, who commands both strong support and deep opposition, says our correspondent.
He has given extra power to anti-corruption agencies and purged former communists, while promoting an assertive foreign policy and traditional Catholic values.
Among his supporters, Andrzej Sulkowski said he voted for Law and Justice "because this party is telling the truth and doing something".
"In their two years of government they did what they could," he told the Associated Press news agency.
But one of Mr Tusk's supporters, Jan Zawisz, said he "didn't like being talked down to for the last two years". ( BBC )