Amazon.com Inc's websites in Britain, France, Germany and Spain suffered an outage for more than half an hour on Sunday night, but it was not immediately clear whether it was due to a cyber attack.
Amazon was among the first U.S. firms to pull the plug on WikiLeaks since it began publishing thousands of U.S. diplomatic cables, withdrawing hosting services last week after being questioned by the U.S. Senate Homeland Security Committee.
A loose grouping of activists operating under the name Anonymous had urged an online attack to crash the amazon.com site by overwhelming it with requests from users.
Amazon.co.uk, amazon.de, amazon.fr and amazon.es were all down for more than 30 minutes until around 2145 GMT when they appeared to work normally again. Amazon.com's U.S. website was unaffected.
Amazon, which operates one of the world's biggest web-hosting businesses as well as a huge e-retail store, had no immediate comment.
The activists briefly brought down the sites of credit-card giants MasterCard and Visa which had stopped processing donations to WikiLeaks.
On Saturday, Anonymous said it had changed its strategy and would now focus on spreading snippets of the leaked cables far and wide rather than on cyber attacks.
On Sunday, neither the blog site that Anonymous had been using for its public statements nor the Internet Relay Chat (IRC) chat channel that organizers had been using was available.
No immediate claim of responsibility for the Amazon outage from pro-WikiLeaks activists was seen on the Twitter website.
(Reporting by Michel Rose, Georgina Prodhan and Alexandria Sage; Editing by Jon Hemming)