Bronislaw Komorowski, Poland's acting president, appears to be heading for victory in a close presidential election after hours of uncertainty.
He now has 52.6%, with rival Jaroslaw Kaczynski on 47.4%, with 95% of votes counted.
Mr Kaczynski had initially accepted defeat, but Mr Komorowski's camp remained cautious.
Mr Kaczynski is the twin of former President Lech Kaczynski, who died in a plane crash with 95 others in April.
This run-off election was held after neither candidate gained more than 50% in the first round in June.
Poland's electoral commission says final results will not be announced until Monday afternoon.
Mr Komorowski is from the ruling Civic Platform party, while Mr Kaczynski represented the main opposition Law and Justice party.
The election has been dominated by a catastrophic plane crash. Poland's first couple - along with other leading political and military figures - died when their plane came down in Smolensk on 10 April as they flew to attend a memorial ceremony for the World War II Katyn massacre.
In an address to supporters earlier on Sunday evening, Mr Komorowski appeared optimistic about his chances of victory.
"Tonight we will open a small bottle of champagne and tomorrow we will open a big bottle," he said.
"We thank everybody - the more so that it was an unusual campaign, a difficult campaign held in the shadow of catastrophe," he added.
Mr Kaczynski, who was prime minister in 2006-2007, initially admitted defeat and said the elections were a "great rehearsal" for regional polls later this year and parliamentary elections in 2011.
"We have to continue changing Poland. We have to continue to be mobilised, we must win," he said.
Rare stability
He paid tribute to his brother, and others who died in the crash, saying: "A movement has emerged from their martyrs' death.
"It was due to the work and service of my brother that a new quality in Polish public life emerged, a return to value, a return to patriotism, everyone in the campaign had to adhere to that," he said.
Mr Kaczynski is seen to have ridden a wave of public sympathy after the tragedy and his popularity has grown considerably.
The BBC's Adam Easton in Moscow says that if Mr Komorowski does win, it will mean a rare period of political stability for the country, with the prime minister and president from the same party.
Mr Komorowski's party favours market reforms and engagement with Poland's European Union partners.
As president, he would be unlikely to use his power of veto over the government's plans to introduce structural and economic reforms, our correspondent adds.
Mr Komorowski won 41.5% in the first round and Mr Kaczynski 36.5%. Eight other candidates were eliminated.
