A European Parliament spokesperson said the outage was linked to an external DDoS attack but did not name a possible culprit.
The European Parliament's official website was down due to a "sophisticated cyberattack" on Wednesday afternoon, mere hours after MEPs passed a strongly-worded resolution declaring Russia a "state sponsor of terrorism."
"The (European Parliament) is under a sophisticated cyberattack. A pro-Kremlin group has claimed responsibility," said European Parliament President Roberta Metsola.
"Our IT experts are pushing back against it & protecting our systems. This, after we proclaimed Russia as a State-sponsor of terrorism."
"My response: #SlavaUkraini," Metsola added, referring to the slogan that means "glory to Ukraine."
The European Parliament's chief spokesperson, Jaume Duch, said the outage was due to "high levels of external network traffic," without naming a potential culprit.
"This traffic is related to a DDOS attack (Distributed Denial of Service) event," Duch added.
Hackers use malicious DDoS attacks to flood networks with high volumes of data that they cannot handle, resulting in normal traffic being disrupted or the network being totally paralysed.
The total duration of the outage was unclear, but it was first detected after the Russian vote, which took place in the early afternoon.
By 15:30 CET, the website became accessible but by 16:00 CET, it went down again.
The Parliament's multimedia centre, which operates as a distinct site, was not affected.
Lawmakers rapidly took to Twitter to condemn the cyberattack, putting the blame squarely on Russia.
"Today's cyberattack on our free institution shows Russia's contempt for democracy extends beyond its own borders. Putin's hackers will not silence us or interfere with our work," Renew Europe, the liberal political group, said in a statement.
"Anti-democrats trying to disturb the decision-making of a democratic institution. We continue our session and stand with Ukraine," said Terry Reintke, co-chair of the Greens group.
"Putin's hybrid war continues," said the European Conservatives and Reformists (ECR) group.
The European Commission also criticised the attack.
In the resolution adopted on Wednesday, MEPs denounced Russia for the "brutal and inhumane" acts inflicted upon Ukraine and its citizens since the launch of the invasion.
"The deliberate attacks and atrocities carried out by the Russian Federation against the civilian population of Ukraine, the destruction of civilian infrastructure and other serious violations of human rights and international humanitarian law amount to acts of terror against the Ukrainian population and constitute war crimes," lawmakers said in a non-binding but highly symbolic resolution.
"In the light of the above, (the European Parliament) recognises Russia as a state sponsor of terrorism and as a state which uses means of terrorism."
Under current EU law, the bloc cannot officially designate an entire country as a terrorist state. Only specific individuals and entities can be legally blacklisted.
Reacting to the resolution, Russian Foreign Ministry Spokesperson Maria Zakharova, who is under EU sanctions for spreading pro-war propaganda, said on Telegram that she would propose "designating the European parliament as a sponsor of idiocy."