The site appears to have responded by switching its main hosting base from Sweden to the U.S., making it available again.
On Tuesday, traffic to the site went to Amazon.com Inc.'s server-for-rent service, based in the U.S.
The site, which distributed a trove of U.S. diplomatic documents on Sunday, said in a Twitter message on Tuesday morning that it was under a "distributed denial of service attack," a method commonly used by hackers to slow down or bring down sites. WikiLeaks didn't identify the attackers.
The site, which is devoted to releasing anonymously submitted documents, also came under attack Sunday, but Tuesday's attack appeared to be more powerful.
Calls to Amazon.com were not immediately returned. Bahnhof, the Swedish Internet company that has been WikiLeaks' main host, had no immediate comment on Tuesday.
In a typical denial-of-service attack, remote computers commandeered by rogue programs bombard a website with so many data packets that it becomes overwhelmed and unavailable to visitors. Pinpointing the culprits is difficult.
Leaked Docs: Karzai Freed Connected Drug Dealers
Wikileaks: China's Influence In N. Korea Limited
Post-9/11 Intel Sharing at Risk after WikiLeaks
Leaked Memos Envision North Korea Collapse
Keeping Secrets, Even From WikiLeaks
U.S. Opens WikiLeaks Criminal Investigation
Seoul: N. Korea to Collapse, China "Incompetent"
WikiLeaks said the malicious traffic was coming in at 10 gigabits per second on Tuesday, which would make it a relatively large effort. According to a study by Internet security company Arbor Networks, the average denial of service attack over the past year was 349 megabits per second, 28 times slower than the stream Wikileaks reported.
Sunday's attack didn't stop the publication of stories based on messages leaked from the U.S. State Department in several major newspapers. WikiLeaks had given the media outlets prior access to the diplomatic cables to publish in conjunction with their Sunday release on its site.
The cables, many of them classified, offer candid, sometimes unflattering assessments of foreign leaders, ranging from U.S. allies such as Germany and Italy to other nations like Libya, Iran and Afghanistan.
What's in WikiLeaks Documents?
WikiLeaks Impacts Washington
Clinton: Document Leak an Attack on U.S., World
U.S. Weighs Legal Options in Wikileaks Release