MasterCard told CNET Monday that it was blocking payments because WikiLeaks is engaging in illegal activity, while Visa joined the blockade Tuesday morning, according to AP.

"MasterCard rules prohibit customers from directly or indirectly engaging in or facilitating any action that is illegal," spokesman Chris Monteiro told CNET.

Similar justifications have been aired by Amazon and PayPal, as corporate America joins in the condemnation of the organisation as it continues to embarrass the US political elite by publishing leaked diplomatic cables.

The pressure on WikiLeaks is likely to continue, with the incoming chairman of the House Homeland Security Committee calling for the operation to be branded as a terrorist organisation, which would make it illegal for US banks to process payments on its behalf. Swiss authorities have already closed a bank account set up at Swiss Post Finance by WikiLeaks founder Julian Assange, who is being pursued over rape allegations in Sweden.

Assange appeared at City of Westminster Magistrates Court today, after handing himself in to the Metropolitan Police Service's Extradition Unit at 09.30 this morning. He was refused bail, and will be held in custody until a court appearance on 14 December.

Wikileaks is not without its supporters. Anonymous, a group of Internet activists and hackers took credit on Monday for crashing the Web Site of Swiss Post Finance and has also launched similar cyber-attacks against PayPal under the banner 'Operation Payback'. Ironically, the cyberwarfare is being promoted over micro-blogging site Twitter, which itself has been pilloried by the freedom of speech movement for effectively erasing WikiLeaks from its top trending items.