The big centre-right and centre-left blocs in the European Parliament have lost their combined majority amid an increase in support for liberals, the Greens and nationalists.
The centre-right European People’s Party remains the largest bloc, and is expected to form a pro-EU coalition.
The Liberals and Greens had a good night, while nationalists were set for victory in Italy and France.
Turnout was the highest for 20 years, bucking decades of decline.
Populists gained ground in some countries but fell short of the very significant gains some had predicted.
In the UK, the newly-formed Brexit Party claimed a big victory, and a strong performance by the Liberal Democrats came amid massive losses for the Conservatives and Labour.
Analysts said the European People’s Party (EPP) was likely to form a “grand coalition” with the Socialists and Democrats bloc, with support from liberals and the Greens.
The turnout bucked a long trend of decline in voter numbers, rising to just under 51% of eligible voters across the 28 member states.
The European Parliament helps shape EU legislation and the results will play a big part in who gets the key jobs in the European Commission, the Union’s executive.