The 4chan image board and forum, which has been used by some members of the Anonymous group of online activists, suffered a distributed denial of service attack on Tuesday.
The site was temporarily overwhelmed with malicious web traffic, according to a post by 4chan founder Christopher 'Moot' Poole.
"Another day, another DDoS," the post said.
The Anonymous group last year launched a series of denial of service attacks in support of whistleblower site Wikileaks. Security company Netcraft said on Tuesday that the motive for attacks against 4chan it is not known.
"Many members of Anonymous inhabit [4chan's] boards, although it is unknown whether the current attack is related to any of the previous DDoS attacks purportedly carried out by Anonymous," Netcraft developer Paul Mutton said in a blog post.
On Monday, Anonymous attacked various Tunisian government websites, according to security firm Sophos. Members of the group gathered on an internet relay chat (IRC) forum on Sunday to pick targets, Sophos senior technology consultant Graham Cluley said in a blog post.
"Impacted websites that appear to have been brought down by the distributed denial of service (DDoS) attack include that of Tunisian president Zine El Abidine Ben Ali and the government's official website," said Cluley.
Anonymous said in an open letter (PDF) that it had launched 'Operation Tunisia' in response to censorship of dissident bloggers and the suppression of reports about Wikileaks on news websites by the Tunisian government.