Vladimir Putin today led a growing band of international leaders voicing support for WikiLeaks' boss Julian Assange, describing his detention in Britain as "undemocratic".

The Russian prime minister's broadside came as hackers escalated their cyber war on opponents of the whistleblower website, setting their sights on Amazon.com.

"Why was Mr Assange hidden in jail  Is that democracy  As we say in the village: the pot is calling the kettle black," Putin said in response to a question on Russia's undemocratic image in US embassy cables leaked by the website.

His comments echoed Brazil's President Luiz Inacio Lula da Silva, who expressed "solidarity" with Assange, blasting the Australian activist's arrest as a blow against "freedom of expression."

Assange has "exposed a diplomacy that had appeared unreachable," said Lula, who criticized the failure of other governments to challenge Assange's detention.

"They have arrested him and I don't hear so much as a single protest for freedom of expression," he said.

Anonymous, the loose-knit group of hackers behind the cyber attacks, announced an assault on the Amazon website for 1600 GMT as part of "Operation Payback."

The move appeared to be part of a developing tit-for-tat cyber conflict targeting companies in reprisal for withdrawing from doing business with WikiLeaks.

Amazon last week booted WikiLeaks off its servers, saying the company had violated its terms of service.

Visa, Mastercard and PayPal earlier suffered disruption to their websites in retaliation for their decision to stop accepting payments for the whistleblower, which rocked global diplomacy last week by dumping 250,000 secret US embassy cables onto its website.

Navi Pillay, the UN High Commissioner for Human Rights, hit out at pressure being exerted on "private companies, banks and credit card companies" to cut commercial ties to WikiLeaks.