A sombre election season in Poland was prolonged by two weeks when a first round of voting produced no immediate successor to Lech Kaczynski, the president killed more than two months ago in a plane crash.
Results show the interim president and parliamentary speaker, Bronislaw Komorowski, is leading Kaczynski's identical twin, Jaroslaw Kaczynski.
But Mr Komorowski appeared to fall short of the 50% needed for outright victory.
The two leaders must now go head-to-head in a runoff vote on July 4, without eight other candidates who ran yesterday.
Mr Kaczynski, 61, addressing supporters, made a rare reference to the plane crash that killed his brother, noting that the campaign has been anything but normal.
He said "it's an election which is the result of a huge catastrophe, a huge misfortune, a huge tragedy".
An exit poll by Millward Brown SMG/KRC showed Komorowski with 45.7% of the vote and Mr Kaczynski with 33.2%. A second poll, by OBOP, showed 41.2% for Mr Komorowski and 35.8% for Mr Kaczynski.
But as partial official results were reported several hours later, it appeared that Mr Komorowski's lead was narrower than it originally seemed.
Based on 71% of voting stations reporting, Mr Komorowski had 40.1% of the votes and Mr Kaczynski had 37.4%, the State Electoral Commission said.
Full official results are expected later today.
Mr Komorowski, 58, told his supporters at a campaign night party in Warsaw that he felt "happy and fulfilled" knowing he has the "support and the trust of millions of voters in Poland.
"In life as in football, overtime is the most difficult. Let's not forget that and let's mobilise all our forces for the grand finale on July 4," he said.
The centre-left candidate, Grzegorz Napieralski, was in third place, and the final outcome will hinge to a large degree on where his votes go. Partial official results showed that Mr Napieralski has nearly 14% support.
Based on more than 94% of voting stations reporting, Mr Komorowski had 41.22% of the votes and Mr Kaczynski had 36.74%, the State Electoral Commission said. Turnout there was 54.85%.