Eurosceptic and far-right parties have seized ground in elections to the European parliament, in what France's PM called a "politicalearthquake".
UK Independence Party and French National Front both performed strongly. The three big centrist blocs all lost seats, though still hold the majority.
The outcome means a greater say for those who want to cut back the EU's powers, or abolish it completely.
UK PM David Cameron said the public was "disillusioned" with the EU.
Mr Cameron said their message was "received and understood".
French President Francois Hollande has called an urgent meeting of his cabinet, as Prime Minister Manuel Valls promised tax cuts a day after the results which he described as "a shock, an earthquake".
Chancellor Angela Merkel - whose party topped the poll in Germany - described the far right victories as "remarkable and regrettable" and said the best response was to boost economic growth and jobs.
Jose Manuel Barroso, outgoing president of the European Commission, stressed that the pro-EU blocs still had "a very solid and workable majority".
He said a "truly democratic debate" was needed to address the concerns of those who did not vote, or "voted in protest".
- Italy In final results, centre-left PM Matteo Renzi (above) scores strong 40%, fending off ex-comic Beppe Grillo's anti-establishment Five Star with 21%, and ex-PM Silvio Berlusconi's Forza Italia with 17%
- France National Front storm to victory - 25%, 24 seats; Centre-right UMP 21%; President Hollande's Socialists a poor third with 14% - lowest ever EP score
- Britain Eurosceptic UKIP in first place, with 27%, Conservatives on 24% and Labour about 25%, Greens beating Lib Dems
- Germany Angela Merkel wins another election - 35% for her Christian Union, 27% for the centre-left SPD. Eurosceptic AfD score strong 7%
- Greece Partial results show far-left Syriza on 26%, PM Antonis Samaras' New Democracy on 23%. Far-right Golden Dawn set to get three MEPs, with 9%. Syriza leader Alexis Tsipras has asked the Greek president to call early national elections.