By Stanislaw Skrzydelski and Piotr Bujnicki
WARSAW (Reuters) - Opposition candidate Jaroslaw Kaczynski unexpectedly nudged into the lead in Poland's presidential race early on Monday, according to partial results, hours after he conceded defeat and his rival claimed victory.
Kaczynski, leader of the right-wing main opposition party Law and Justice (PiS), won 50.4 percent of the vote against 49.6 percent for the centrist ruling party's candidate Bronislaw Komorowski, the results based on 51 percent of the vote showed.
The election commission was expected to update the tally shortly. Final official results were not due until late Monday.
"We still have good hopes but we need to wait. We are going head to head," Elzbieta Jakubiak from Kaczynski's camp told TVN television.
A slew of exit polls published immediately after voting ended on Sunday had all given Komorowski, who is Poland's acting president, a lead of up to six percentage points.
"I must begin as good manners require by congratulating the winner, Bronislaw Komorowski," Kaczynski told his supporters after the exit polls showed him losing.
Komorowski gave a victory speech at his headquarters by saying he would work for the unity of all Poles.
Kaczynski is the identical twin brother of President Lech Kaczynski, whose death in a plane crash in April precipitated the election.
Investors would prefer a Komorowski victory because he and Prime Minister Donald Tusk are close allies from the same party, and the new president is expected to work smoothly with the market-oriented government.
In Poland, the government led by the prime minister sets policy, but the president can propose and veto laws, appoints many key officials and has a say in foreign and security policy.
Lech Kaczynski vetoed several government bills before his tragic death in the plane crash in Russia on April 10.
Markets would view a Jaroslaw Kaczynski win with concern because of his opposition to spending cuts and privatization and the likelihood of increased conflict between government and president.
The European Union's largest ex-communist member is the only economy in the 27-strong bloc to have avoided recession last year, but a sharp slowdown hammered tax revenues and drove up the budget deficit to 7 percent of gross domestic product.
(Writing by Gareth Jones, editing by Mark Trevelyan)