European leaders are reeling from the blow delivered to the EU project by voters who abstained in droves or protested by backing Eurosceptics.
The disappointing turnout figures have spread gloom in Brussels
French President Jacques Chirac said the EU elections were "disappointing for all of us and for Europe".
Governing parties in Germany, France and Poland suffered big losses, while many Eurosceptic parties had their best result yet in the EU Parliament polls.
The grim message from voters comes amid tough talks on a new EU constitution.
Irish Foreign Minister Brian Cowen, chairing a meeting of his EU counterparts in Luxembourg on Monday, warned that the "political credibility" of the EU was at stake.
"We need to show that Europe works," he said.
His words were echoed by UK Foreign Secretary Jack Straw, who said voters "want a European Union that works better in their interests".
The BBC's Tim Franks in Luxembourg reports that the UK is one of several countries with reservations about parts of the draft EU constitution.
Even if a consensus is reached by EU leaders this week, the elections have shown how tough it will be to gain approval for the constitution in a referendum, he says.
Turnout reached a record low, with just 45.3% of EU voters casting ballots.
East European apathy
European Parliament spokesman David Harley said turnout was "pathetically low" in the 10 new member states, which averaged a mere 26.4%.
The lowest turnout was in Slovakia, where fewer than 17% cast their votes.
The figure was only slightly higher in neighbouring Poland, where President Aleksander Kwasniewski described his country's apathy as "a disease we will have to look at". He said there was a need to analyse "why we are so far from civic values".
Outgoing European Parliament President Pat Cox said that "regrettably, Europe is too absent from European elections in east and west," and EU citizens needed to be taught more about EU institutions.
The BBC's European affairs correspondent William Horsley says the protest vote across the EU may push the heads of government to water down some of their integrationist ambitions.
Germany's Chancellor Gerhard Schroeder blamed the election debacle on the pace of EU enlargement and the fundamental reforms under way in Europe - but he said both were essential and "those who govern Europe have similar difficulties".
Germany's governing Social Democrats recorded their worst result since World War II. Official results showed the party took just 21.5% of the vote, with the Christian Democrats emerging as clear winners with 44.5%.
- French President Jacques Chirac's Union for the Popular Movement, with 16.6% of the vote, finished a far second behind the Socialist Party, on 28.9%, according to the final results.
- In Britain, Labour and the Conservatives both saw their vote slump. With most results in, the two main parties looked set to secure less than half of the vote between them, for the first time ever. The Tories had 27.4% and Labour 22.3%, while the Eurosceptic UK Independence Party was running third with nearly 17%.
- In Poland, largest of the new EU members, partial results indicated the anti-EU League of Polish Families came second with 16.4%, while the governing left party won just 9%.
- Italian Prime Minister Silvio Berlusconi suffered a setback, with his Forza Italia party dropping to 21%, compared with 29.4% in the 2001 general elections. But partners in his centre-right coalition made modest gains.
- In Spain, the ruling Socialists appear to have bucked the trend, winning some 43.7%, while the conservatives took about 40.8 %, according to provisional results.
- With all the ballots counted in the Czech Republic, Eurosceptic Civic Democrats won 30% of the vote to trounce the ruling Social Democrats, who only managed to poll 8.8%.
The EU Parliament will have its first Roma deputy - Livia Jaroka - thanks to voters in Hungary. She says she will represent all of Europe's 12 million Roma.
In all, just over 44% of the nearly 350 million eligible voters in the 25 member states cast their ballots, making it one of the biggest democratic exercises in the world. Voting was spread over four days.
The overall balance of the 732-member parliament was little changed, with the centre-right European People's Party remaining the biggest bloc, followed by the Socialists.