Russian Prime Minister Vladimir Putin Thursday led growing support from some world leaders for the beleaguered WikiLeaks founder, describing his detention in Britain as "undemocratic".

The wave of support for Julian Assange, currently in a British jail as Sweden seeks his extradition on rape charges, came as hackers -- dubbed "hack-tivists" -- stepped up cyber attacks on those opposed to WikiLeaks.

But after taking down the websites of Visa, Mastercard and others, supporters of the whistleblower website tried but failed to knock online retail giant Amazon.com offline.

The loose-knit group of hackers known as "Anonymous" said they would attack the Amazon website as part of what they are call "Operation Payback."

But the Amazon.com website did not appear to have experienced any downtime, and Anonymous acknowledged defeat on another Twitter feed.