Pro-EU Warsaw Mayor Rafal Trzaskowski is set to face historian Karol Nawrocki in Poland's presidential runoff on June 1, an exit poll showed after the first round.
An overall victory for Trzaskowski would be a boost for the centrist government led by former European Council chief Donald Tusk, which has been at loggerheads with the current president.
A victory for Nawrocki would likely extend the political deadlock and analysts predict that fresh parliamentary elections may have to be called.
Trzaskowski was expected to get 31.2 percent of the vote and Nawrocki 29.7 percent in Sunday's first round, according to an Ipsos poll based on 90 percent of the votes. Preliminary results are expected early Monday.
"This result shows... how determined we have to be, how much work lies ahead of us," Trzaskowski, 53, told supporters in the southeastern town of Sandomierz.
Nawrocki, 42, struck a defiant tone: "We must win these elections to prevent one political camp's monopoly of power," he said.
The far-right in Poland with its two candidates -- multi-millionaire entrepreneur Slawomir Mentzen and MEP Grzegorz Braun -- tipped to receive more than 20 percent between the two of them.
"This is the greatest success in the history of our political camp," Mentzen said.
Wojciech Przybylski, head of the Res Publica foundation, said "Trzaskowski can't be absolutely sure of winning".
"The runoff result will depend on turnout and mobilisation among youths who voted for Mentzen" and candidate Adrian Zandberg, he told AFP.
Wojciech Kuras, 44, said he did not vote for Trzaskowski but might in the runoff.
"It will be hard for Rafal Trzaskowski. This is such a difficult result because if others join together it can be equal," said the entrepreneur from Sandomierz.
Tusk said an "all-or-nothing" battle had begun.
"It will be a tough fight for every vote. These two weeks will decide the future of our homeland," the prime minister said on X.
Political scientist Ewa Marciniak said the electoral campaign would be "brutal".
"Each candidate will try to discredit his rival at all costs, by any means necessary," she told AFP.
So far, campaigning in the EU and NATO member has largely revolved around foreign policy, showcasing a clash of philosophies over Poland's engagement with the EU and the United States.
The key to the election could be whether supporters of Mentzen, who came third in the exit poll with 14.5 percent, back Nawrocki in the second round.
Poland's head of state is commander-in-chief of the armed forces, steers foreign policy and can introduce and veto legislation. The result will be closely watched across Europe, where Poland has increased its influence under Tusk.
