Although pro-European Union parties are expected to retain the majority of the 751 seats in the new legislature, anti-EU and far-right parties made significant electoral gains.
In France, 25 percent of voters favored the far-right National Front, prompting French President François Hollande to call a "crisis meeting" on Monday.
Voting in Denmark, Greece, and the United Kingdom also boosted so-called Euroskeptic parties, who oppose the EU.
How will the rise of Euroskeptic groups impact the EU?
According to European politics expert Simon Usherwood, who spoke to CNN about the election results, "They don't have enough votes to stop legislation going through but what they will get particularly on the far right, is the time for speaking in debates, the chairmanship of certain committees, which means that they're going to have much more of a platform on which they can sell their message to voters."
More than 43% of the almost 400 million Europeans eligible to vote cast ballots in the European parliamentary elections.