Users across Europe were unable to access the site for several hours.
The site appeared to be online in the UK by Saturday afternoon.
“Wikipedia was hit with a malicious attack that has taken it offline in several countries for intermittent periods,” a Wikimedia Foundation spokesperson said.
The charitable foundation operates the website.
“The attack is ongoing and our Site Reliability Engineering team is working hard to stop it and restore access to the site,” the spokesperson said, early on Saturday.
A DDOS attack occurs when a huge network of computers all try to access a certain website or internet service at the same time, causing it to collapse under the strain of too much traffic.
These networks, or ‘botnets’, can be made up of tens of thousands of computers, which have been compromised by malicious hackers without the knowledge of their owners.
“As one of the world’s most popular sites, Wikipedia sometimes attracts “bad faith” actors,” the Wikimedia spokesperson added.
“Along with the rest of the web, we operate in an increasingly sophisticated and complex environment where threats are continuously evolving.
“We condemn these sorts of attacks.
“They’re not just about taking Wikipedia offline. Takedown attacks threaten everyone’s fundamental rights to freely access and share information.
“We in the Wikimedia movement and Foundation are committed to protecting these rights for everyone.”
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The online encyclopedia is one of the world’s most widely used websites.
There are Wikipedia sites in 300 different languages, with some 46 million articles accessed by 1.4bn unique devices every single month, according to figures from 2018.