Spain's centre-right Popular Party was one of the night's big winners
Centre-right parties have done well in elections to the European Parliament at the expense of the left.
Far-right and anti-immigration parties also made gains, as turnout figures plunged to 43% - the lowest since direct elections began 30 years ago.
The UK Labour Party, Germany's Social Democrats and France's Socialist Party were heading for historic defeats.
The centre-right European People's Party (EPP) looks set to continue to hold power in the parliament.
Jose Manuel Barroso, who seems set for a second term as European Commission president following the centre-right success, thanked voters and assured them their voices would be heard.
"Overall, the results are an undeniable victory for those parties and candidates that support the European project and want to see the European Union delivering policy responses to their everyday concerns," he said.
Socialist leader Martin Schulz said his group's defeat would be analysed.
"It's a sad evening for social democracy in Europe. We are particularly disappointed, [it is] a bitter evening for us," he said.
German Chancellor Angela Merkel praised the EPP's performance, saying: "This result shows that the core of society in Europe has become stronger."
Vice-president of the European Commission Margot Wallstrom said the low turnout was a "bad result".
Fringe groups appear to have benefited, with far-right and anti-immigrant parties picking up seats in the Netherlands, Austria, Denmark, Slovakia and Hungary. The British National Party won two seats - its first ever in a nationwide election.
Greens also made gains - the Green-European Freedom Alliance bloc has so far taken 50 seats, compared with 43 in the last assembly.
Sweden's Pirate Party, which wants to legalise internet file sharing, won 7% of the national vote and one of the country's 18 seats in the European Parliament.
Several governments battling the economic downturn are facing a heavy defeat, says the BBC's Oana Lungescu in Brussels.
However, governing parties in France and Germany appear to have done relatively well despite the crisis.
Angela Merkel described the increase in the vote of her Christian Democrats over the Social Democrats as "sensational" and said it boded well for her chances in the nation's general election in September.
In results so far:
- French President Nicolas Sarkozy's UMP trounced socialist opponents, while greens from the Europe-Ecologie party also made gains
- In Italy, Prime Minister Silvio Berlusconi's centre-right People of Freedom party won most votes - 35% - although that was well below his prediction. The anti-immigrant Northern League made strong gains
- In the UK, the governing Labour Party suffered a serious defeat, gaining its lowest share of the vote for a century
- Spain's conservative Popular Party beat the ruling Socialists, but the four percentage point margin was lower than they had expected
- Poland's governing centre-right Civic Platform gained ground at the expense of the Eurosceptic Law and Justice Party
- Portugal's conservative Social Democrats secured about 31% of the vote. The ruling Socialists fell a massive 18 percentage points from the last European election, to about 26%
- Austria's far right increased its vote on the last European election but was well down on its percentage in last year's national polls
- Greece's Socialist party, PASOK, bucked the European trend by securing the largest vote percentage, ahead of the ruling conservatives
Voters have been choosing representatives mainly from their own national parties, many of which then join EU-wide groupings with similarly-minded parties from other countries.
Hungary's Jobbik party was one of several far-right groups to do well
The centre-right EPP retains its place as the largest grouping over the past five years, securing an estimated 264 of the 736 seats (the overall number of seats in the assembly has been reduced from 785). The socialists are on 183, while the liberal ALDE has an estimated 84.
Provisional figures released by the EU suggested turnout was at an all-time low in some countries, including France, where it dropped to 40.5%.
Lowest turnout was seen in Slovakia (19.6%) and Lithuania (20.9%), while the highest figures came from Luxembourg (91%) and and Belgium (85.9%) - both countries where voting is compulsory.
Overall turnout has fallen at each European election in the last 30 years, from a high of nearly 62% in 1979.