The European Parliament has elected a new president in a power-sharing deal between its two most powerful groups.
Josep Borrell was elected president of the parliament in the first round
Spanish socialist Josep Borrell will lead the assembly before stepping down halfway through his five-year term in favour of a conservative candidate.
He pledged to try to build unity, but various political blocs criticised the deal that brought him to power.
The first meeting of the newly enlarged parliament will vote on a European Commission leader later this week.
Mr Borrell, 57, won an outright majority of the 732 MEPs from 25 countries in the first round of what was described as a tense ballot.
"Thank you to those who have voted for me," he told the inaugural meeting of the new parliament in its Strasbourg home.
Correspondents say he will need to try to rebuild confidence in EU institutions after a low turnout in the continent-wide June elections that followed the union's expansion to 10 new member states.
"We need to build this institution... speaking a language people can understand," he told the assembly.
Mr Borrell secured victory with 388 votes, ahead of former Polish dissident Bronislaw Geremek, who won 208 votes, and French communist Francis Wurtz, who was supported by 51 MEPs.
His success was expected after a deal between the conservative European People's Party and the Party of European Socialists - the two largest groups in the parliament.
The centre-right EPP bloc agreed to support Mr Borrell for the first two-and-a-half years in return for him stepping down in favour of their candidate, Germany's Hans-Gert Poettering, for the second half of the term.
While Mr Borrell won ringing tributes from supporters, Liberal group leader Graham Watson criticised the power-sharing deal between parties from the left and right.
"The alliance they have formed is an unnatural one," he said.
"I believe the citizens of the European Union are fed up with back-room deals."
All the candidates for president were opposed by Eurosceptic parties, including the UK Independence Party, which has vowed to "wreck" the parliament.
The new MEPs were welcomed by the parliament's oldest deputy
Its parliamentary leader, Nigel Farage, ceremonially ripped up his ballot paper in a news conference on the sidelines of the session.
Later this week the parliament is expected to confirm former Portuguese Prime Minister Jose Manuel Durao Barroso as president of the European Commission, the EU's executive arm.
Mr Barroso, who emerged as the compromise choice of EU national leaders to replace Romano Prodi, will address the parliament on Wednesday before a scheduled vote on his appointment on Thursday.
The BBC's William Horsley in Strasbourg says there is a sense of history as the politicians from 25 European countries, including the 10 new member states, hold their inaugural three-day meeting.
The parliament - which represents 450 million people and now uses 20 official languages - was opened by its oldest deputy.
The 79-year-old Italian MEP, Giovanni Berlinguer, welcomed the new members.
"Europe's borders have been expanded, and the union's activities are also going to be expanded," he said.