Polish daily Rzeczpospolita reports that older people living in rural areas with a basic level of education formed the backbone of Mr Kaczynski's support.

Turnout was among the lowest in 15 years at just 50 percent.

Mr Kaczynski is a 56 year-old lawyer, former anti-communist activist and mayor of Warsaw as well as a newcomer to the European political scene.

His twin brother, Jaroslaw Kaczynski, is the head of Law and Justice which also won the parliamentary elections in September.

The Polish president has limited powers but is the official chief of the army, giving him a strong role in foreign affairs.

Turmoil possible

The result might damage Law and Justice and Civic Platform's efforts to form a coalition government.

Mr Kaczynski made conciliatory remarks to Mr Tusk following the news, offering him the post of speaker in the lower house, while the party's prime ministerial candidate, Kazimierz Marcinkiewicz, said he plans to forge a government deal by Friday.

But Civic Platform member Bronislaw Komorowski said his party might be better off making a strong opposition to Law and Justice instead.

The two parties each hold about a quarter of the seats in parliament, with newspaper Gazeta Wyborcza explaining that if Civic Platform was to block the government's plans, it could force an early second election.

The parties are split on the economy, with Law and Justice pushing for better welfare and delayed eurozone entry, while Civic Platform favours tighter fiscal policy and euro entry in 2010.

Financial analysts predict the zloty and Polish government bonds will weaken on Monday, Reuters and Bloomberg reported.

European values at risk?

Poland's swing to the right could also complicate relations with EU socialists on the issue of European values.

Law and Justice ran on an anti-corruption ticket with Roman Catholic and eurosceptic overtones, scoring pre-election points by criticising gay rights campaigners and talking positively about the death penalty, which is illegal in the EU.

As mayor, Mr Kaczynski tried to block a gay pride march in Warsaw in June, while Mr Marcinkiewicz recently told Newseek that homosexuality is "unnatural".

The leader of the European Parliament's socialist MEPs, Martin Schulz, described the remark as "shocking".

But conservative French president Jacques Chirac was among the first to congratulate Mr Kaczynski, inviting the winner to visit Paris in early 2006.

"I am sure that under your leadership Poland will further assert its role in Europe", Mr Chirac said.