Boris Tadic, a pro-western politician, has been sworn in as the new president of Serbia after an 18-month spell during which the post was left vacant.
Boris Tadic has promised to bring Serbia closer to Nato and the EU
He is the country's first non-communist president since World War II.
Security was tight for Mr Tadic, who has pledged to continue the policies of former party colleague and PM, Zoran Djindjic, who was assassinated in 2003.
Mr Tadic's victory over his nationalist rival was seen as an endorsement of his plans to bring Serbia closer to Europe.
The office of president has lain vacant for more than a year as successive elections were annulled because the turn-out was not high enough.
The minimum turn-out requirement was scrapped for the latest poll in June.
Mr Tadic took the oath of office at a formal ceremony in Belgrade.
In the election, Mr Tadic won 53.7% of the vote and his nearest rival, Tomislav Nikolic of the Serbian Renewal Party, polled 45%.
Mr Nikolic had called for Serbia to end co-operation with the Hague war crimes tribunal and return to the nationalist policies of the Slobodan Milosevic era.
While Mr Tadic's scholarly background and his appearance at popular music and sporting events have won him a following among the nation's young, Mr Nikolic continues to command support with rural Serbs, refugees and veterans of the conflicts in Bosnia and Kosovo.
Mr Tadic - a soft-spoken psychologist - represents the reformist movement in Serbian politics which helped topple Slobodan Milosevic in 2000.
Serbia's stagnating economy and its battered international profile have improved little since then.
Addressing parliament after being sworn in, Mr Tadic stressed that Serbia's immediate future "will be marked by its intention to join European Union".
He had earlier pledged to do his utmost to preserve "the sovereignty and territory of Serbia" and "the realisation of human and civic freedom and rights".
But the BBC's Belgrade correspondent Matt Prodger says Mr Tadic will have limited powers as president, and is likely to be drawn into confrontations with Prime Minister Vojislav Kostunica, who is leader of a rival party.