Popular online reference website Wikipedia went down in several countries after the website was targeted by what it described as a "malicious attack".
In a separate statement the Wikimedia Foundation said that the attack on the Wikipedia online encyclopedia -- one of the world's most popular websites -- was "ongoing" and teams were working to restore access.
DDoS attacks often involve legions of zombie computers -- machines infected with viruses and commanded to simultaneously visit a website.
Such a massive onslaught of demand can overwhelm website computer servers, slowing service or knocking them offline.
Wikimedia condemned the breach of its server, saying it threatened "everyone's fundamental rights to freely access and share information."