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 BBC reports.
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.