Bronislaw Komorowski took the oath of office as Poland's new President yesterday and vowed to build national unity.
But the man he beat for the top job boycotted the ceremony in an indication of enduring political tension.
Mr Komorowski, a moderate conservative from the ruling Civic Platform, defeated Jaroslaw Kaczynski in the 4 July run-off of an election triggered by the death of Kaczynski's twin brother, President Lech Kaczynski, in a plane crash in April.
"Poland needs cooperation between the most important institutions of the state, between the government, the parliament, the President, but also between political parties, between those in government and those in opposition," Mr Komorowski said in a speech to parliament after taking the oath. "As President I declare my will for such co-operation."
Jaroslaw Kaczynski, who leads the right-wing main opposition, Law and Justice (PiS), stayed away from the ceremony, as did some of his deputies. Lech Kaczynski, a combative right-winger, and the centrist government of prime minister Donald Tusk were often at loggerheads as the president vetoed some draft legislation.
Asked why Mr Kaczynski did not attend Friday's ceremony, the deputy head of the PiS parliamentary party, Beata Szydlo, told Reuters: "Clearly he has some other things to do."