The European Parliament website was hit by a cyberattack claimed by pro-Russian hackers on Wednesday, shortly after lawmakers approved a resolution calling Moscow a "state sponsor of terrorism."
"The European Parliament is under a sophisticated cyberattack. A pro-Kremlin group has claimed responsibility," the parliament's president, Roberta Metsola, tweeted.
"Our IT experts are pushing back against it and protecting our systems. This, after we proclaimed Russia as a State-sponsor of terrorism. My response: #SlavaUkraini (Glory to Ukraine)," she added.
Spokesperson Jaume Duch said the parliament's website had been targeted by a so-called "DDOS attack" designed to force high levels of outside traffic onto the site's server to disrupt the network.
Another source at the parliament, speaking on condition of anonymity, said the attack was the "most sophisticated in recent history" to target the institution.
The parliament's website went down shortly after EU lawmakers voted overwhelmingly to recognize Russia as a "state sponsor of terrorism" over its ongoing invasion of Ukraine.
European Pirate Party MEP Mikulas Peksa said "there are reports that the pro-Russian hacking group Killnet has claimed responsibility for the attack."
"If these reports are true, this is a massive attack on European democracy that will require further action," he said.
Killnet has in the past claimed attacks on U.S. government websites and said it has taken action against other countries opposed to Russia's invasion of Ukraine.
German MEP Rasmus Andresen said that it was not yet known if the attack was linked to the resolution on Russia, but insisted the parliament's systems were "not sufficiently prepared."
"I hope that today's events will lead us to better protect our data and our democracies because it certainly won't be the last time we are victims of such attacks," he said.
Moscow has repeatedly been accused by Kyiv and its Western allies of carrying out cyberattacks against key targets as it wages war in Ukraine.
The EU has said in recent months that it is looking to bolster its computer defenses.