The newly elected Polish right-wing president Karol Nawrocki will take office today, marking the beginning of a period of possible confrontation with the pro-European government, DPA reported.
The 42-year-old independent historian, who was supported by the opposition Law and Justice (PiS) party, declared victory in the run-off in early June.
He defeated the liberal and mayor of Warsaw, Rafal Trzaskowski - which was seen as a major loss for the government of Prime Minister Donald Tusk.
Nawrocki succeeds Andrzej Duda, who is stepping down after two terms in office.
The Polish head of state is elected for a five-year term and has broad powers. The president is commander-in-chief of the army, has a say in foreign policy decisions and can veto laws passed by parliament.
Duda, who was also supported by PiS, has often used his power to block Tusk's reform agenda.
Nawrocki has already warned that the prime minister can expect "strong resistance from the presidential palace".
Traditionally, the Polish president is sworn in in the National Assembly, which consists of two chambers. The ceremony is scheduled to begin at 10 a.m. local time (11 a.m. Bulgarian time).