The new Polish President, Lech Kaczynski, has taken his oath of office, in a televised ceremony before both houses of parliament.
Lech Kaczynski: Championing traditional Catholic values
Mr Kaczynski, 56, said Poland must be "rebuilt and cleansed". A staunch Roman Catholic, he has pledged to root out corruption and communist influence.
Mr Kaczynski, a former Warsaw mayor elected in October, also vowed to help chart a new course for the EU.
He supports the socially conservative Law and Justice party.
It is led by his twin brother Jaroslaw Kaczynski.
"The state is not working properly," Lech Kaczynski said on Friday. "It must be rebuilt and cleansed."
He called for "a great change in our political life".
On the economy, he said action to ensure fast growth must be balanced by action to tackle social problems, "with unemployment first on the list".
Lech Kaczynski replaces Aleksander Kwasniewski, a former communist who had held office for 10 years.
Law and Justice is firmly opposed to former communists and advocates limited market reform and a robust approach to the European Union.
The Kaczynskis oppose any concessions to gay rights or any relaxation of the law against abortion.
President Kaczynski sought to quell fears that Poland might become isolated, saying he would play "an active role in preparing a new project for the EU after the collapse of the constitutional treaty".
"Our goal is a Union, [defined] as an organisation with lasting, close and institutionalised co-operation between states based on the value of solidarity. We must take big strides."
He said Poland would maintain its strong ties with the US and played down recent tensions with Russia, saying "there exist no objective reasons why relations with Russia should not be good".