Kaczynski conceded defeat late on Sunday when exit polls showed he trailed Komorowski in the race to replace Kaczynski’s twin brother, Lech, who was killed in a plane disaster in April.
With 95.1 per cent of votes counted, the liberal Komorowski had 52.63 per cent and conservative ex-prime minister Kaczynski 47.37 per cent. Final results were due on Monday.
The run-off between Komorowski, 58, and Kaczynski, 61, marked the latest chapter in a bitter power struggle.
Komorowski pledged however to end years of bad blood.
“Divisions are an inseparable part of democracy,” Komorowski said late on Sunday. “But we have work to do to ensure these divisions don’t prevent co-operation.”
Komorowski – who as speaker of parliament became acting head of state after the crash that killed the president – is a key ally in the liberal Civic Platform party of Prime Minister Donald Tusk.
Analysts said the liberals’ win would test the Tusk government.
The liberals have underscored that Poland was alone in the 27-nation European Union in posting economic growth last year but say more needs to be done to plug holes in state coffers.
“Of course there will be no excuse not to push through reforms when Komorowski comes in,” Danske Bank economist Lars Christensen said.
The ballot was a gauge of strength for municipal polls this year and a general election in 2011.
“There’s no longer an alibi. They have 500 days to prove they are capable, that they’re an effective government,” said Warsaw sociologist Edward Wnuk-Lipinski.
“If they prove that, they’ll win the elections. If not, they’ll lose them.”
“Reforms are always tough and unpopular.”
Jaroslaw Kaczynski struggled to shake off his divisive, confrontational reputation and failed to build on the outpouring of sympathy after his twin’s death.
The head of the eurosceptic Law and Justice party was his twin’s premier in 2006-2007 but lost a general election to Civic Platform.
Thereafter, Law and Justice counted on Lech Kaczynski, who used presidential veto powers 18 times to block the liberals’ laws.
With an eye on core conservatives – older, small-town or rural residents, in contrast with younger, urban liberals – the twins battled welfare reforms and a new privatisation drive.
Lech Kaczynski was expected to seek a second five-year term in an election later this year but had trailed Komorowski in polls.
Sunday’s vote was watched closely elsewhere in the EU, which ex-communist Poland joined in 2004, because the Kaczynskis regularly clashed with other leaders.
Jaroslaw Kaczynski’s supporters were left reeling.
“It shouldn’t be like this!” tearful care-worker Mariola Kupiec said.
She had just placed a candle at a makeshift shrine outside Warsaw’s presidential palace in memory of Lech Kaczynski and his wife Maria.
They and 94 other Poles who died when their country’s presidential jet crashed on April 10 in Smolensk, western Russia as a delegation headed for a World War II commemoration.
“We’ll win next year,” said Kupiec.
Kaczynski also looked ahead.
“This was a great rehearsal” he told supporters.
“We have to continue changing Poland: there are elections ahead of us, local and parliamentary. We have to continue to be mobilised, we must win,” he said.
“At this moment, I’d like to mention those thanks to whom we are here: My brother and the others who died in the Smolensk catastrophe,” he added.
“A movement has emerged from their martyrs’ death.”