After the party, extradition fight begins for WikiLeaks founder

Civil rights demonstrators will gather outside the Royal Courts of Justice this morning to support the WikiLeaks founder, Julian Assange, who is fighting a last-ditch battle to avoid extradition to Sweden.

Internet users believed to be linked to the hackers' network Anonymous are also threatening to create havoc with police computers, in what appears to be a planned revenge for the threat to extradite Assange.

A message posted yesterday on Twitter from an account under the name Anonymous Sabu, whose feed has over 7,000 followers, warned: "We are working on two of the biggest releases for Anonymous in the last four years. Put your helmets on. It is war."

It would appear the group is threatening to release hacked data.

Later messages added: "Everyone brace. This is literally explosive," and "get ready for the truth." And another, addressed to the "intelligence community", warned: "Your contractors have failed you. Tomorrow is the beginning."

The demonstrations and online threats reflect a widely shared suspicion that there is a political motive behind allegations of sexual assault on two women made against Assange after a visit to Sweden in August.

Since December, the WikiLeaks founder has been living under house arrest in Effingham Hall, the Norfolk mansion of the journalist Vaughan Smith, founder of the Frontline Club, where Assange threw a lavish party with an A-list celebrity guest list at the weekend to mark his 40th birthday.

The invitation told guests: "You can charter a private plane to Norwich International Airport which has a private air strip and is a half-hour taxi journey from Ellingham Hall, or alternatively you can land helicopters on the Ellingham Hall property on the field in the north-west quadrant of this map."

However, there were no reported sightings of helicopters landing anywhere near Effingham Hall, and the biggest names on the guest list, Brad Pitt and Angelina Jolie, did not turn up.

Craig Murray, the human rights activist and former ambassador to Uzbekistan, blogged about the event just before he set out for it by train. He wrote: "I am going because I think WikiLeaks do essential work and because I think Julian is an extraordinary man and is being stitched up... Nonetheless, I worry that the amusing fact that the invitation tells you where to land your private jet or helicopter, actually is an indication of where WikiLeaks is going wrong."

Swedish police want to question Assange about his relations with two women during a visit to the country last August. He denies sexual assault.

His supporters believe he is the victim of a US government vendetta in retaliation for the publication on WikiLeaks of more than 250,000 secret diplomatic dispatches. The only person charged with any offence over the leak is US Army Private Bradley Manning, who is being held in Fort Leavenworth, Kansas, awaiting trial.

Assange's supporters have been alarmed by comments made by leading US politicians such as Sarah Palin, who called on Facebook for him to be "pursued with the same urgency we pursue al Qa'ida and Taliban leaders".

In January, Assange's legal team claimed in court that he was in danger of being sent to Guantanamo Bay or even executed if he were handed over to the US authorities. He has since appointed a new legal team and will be represented by human rights lawyers Gareth Peirce and Ben Emmerson.