The elections, which are spread over four days in the EU's 28 member states, are set to produce major gains for anti-immigration parties that are bent on dismantling the European Union from the inside.
The vote, for which some 400 million Europeans are eligible to cast their ballots, comes as the EU struggles for relevance in the aftermath of the eurozone crisis and grapples with the chaos on its borders in Ukraine.
Polls opened at 1100 IST in the Netherlands and 1130 IST in Britain. Ireland and the Czech Republic vote tomorrow, Latvia, Malta and Slovakia on Saturday, and the other 21 EU nations on Sunday.
When the results are announced on Sunday, eurosceptic parties may top the polls in Britain, France, Italy and the Netherlands.
The anti-immigration and anti-EU UKIP, led by Nigel Farage, and Geert Wilders' virulently anti-Islam Party of Freedom (PVV) in the Netherlands, are both forecast to make big gains.
The party's rise was seen as a factor in Conservative Prime Minister David Cameron's pledge to hold a referendum on Britain's membership of the EU in late 2017.
Farage, a former commodities trader who likes to hold court with journalists in the pub, has ruled out joining a far-right bloc of Wilders' party and France's National Front, led by Marine Le Pen, saying the National Front is anti-Semitic.
"If we get what we like, things will never be quite the same again," he told reporters.
Farage also predicted the highest turnout in European elections since the first in 1979, despite the fact that the figure dropped from 62 per cent at that vote to just 43 per cent in the last election in 2009.
