WIKILEAKS founder Julian Assange was refused bail by a British judge over alleged sex crimes in Sweden, dealing a fresh blow to the website which vowed to stay online and reveal more secrets.

The elusive 39-year-old Australian said he would fight an extradition request by Swedish authorities as he appeared in court in London just hours after he emerged from a month in hiding and surrendered to police.

Filmmaker Ken Loach, socialite Jemima Khan and campaigning journalist John Pilger each offered to put up part of his bail but a judge in London refused, saying a court would review the situation at a hearing on December 14.

"I am satisfied that there are substantial grounds to believe that if granted bail, he would fail to surrender," district judge Howard Riddle said at City of Westminster Magistrates' Court.

The judge said the Swedish arrest warrant contains "extremely serious allegations" and that Assange had the "means and ability to abscond if he wants to."

The court heard Assange is accused of unlawfully coercing and sexually molesting a woman on August 14, and of deliberately molesting her on August 18.

A fourth allegation claims Assange had sex with a second woman on August 17 while she was asleep at her Stockholm home, and without using a condom.

The WikiLeaks founder, who has denied the allegations, seemed calm as he appeared in court. Wearing a navy blue suit and a white shirt without a tie, he spoke to confirm his name and address in Australia.

British police said earlier that officers had arrested Assange on a European warrant "by appointment at a London police station" at 0930 GMT.

WikiLeaks criticised the court decision to hold Assange as "bizarre", and said it would continue to release documents from a cache of 250,000 confidential US diplomatic cables that it started to publish on November 28.

"Let down by the UK justice system's bizarre decision to refuse bail to Julian Assange. But Cablegate releases continue as planned," it said on Twitter.

Assange's lawyer Mark Stephens told journalists outside the court that the allegations were "politically motivated", adding that he expected a "viral campaign" on the Internet on his client's behalf.

"We have heard the judge say he wishes to see the evidence himself. I think he was impressed by the fact that a number of people were prepared to stand up on behalf of Mr Assange and declare his innocence," he said.

Loach, Khan -- former wife of Pakistan cricket great Imran Khan and one-time girlfriend of film star Hugh Grant -- and Pilger each offered 20,000 pounds (23,600 euros, 31,400 dollars) towards bail. Another three donors offered 120,000 pounds between them.

Australian-born Pilger said the case against Assange was "a travesty", adding: "This is a man who has made some very serious enemies for the best reasons."

The prosecutor's office however came under cyber attack by members of the "cyber hacktivist" group called "Anonymous", in the latest salvo by his online supporters.

And although US Defence Secretary Robert Gates described Assange's arrest as "good news", the State Department insisted it did not concern a separate US probe into the leaked files.

WikiLeaks is battling to stay afloat after infuriating Washington with the release of the cables, which have resulted in a string of diplomatic embarrassments.

In one of the latest, cables released Tuesday showed Saudi Arabia proposed setting up an Arab force to fight Hezbollah militants in Lebanon with the help of the United States, UN and NATO.

WikiLeaks has hopped from server to server as various countries tried to close it down and hackers attacked the site, though its supporters have responded by setting up hundreds of "mirror" sites to keep it online.

The website is coming under increased financial pressure, with Visa following in the footsteps of MasterCard and PayPal Tuesday by announcing it was suspending all payments to WikiLeaks.

Swiss authorities shut down one of Assange's bank accounts on Monday, while a major WikiLeaks donor in Germany is in trouble for not filing its accounts on time.

In an opinion piece for The Australian newspaper after his arrest, Assange said the website was "fearlessly" pursuing facts in the public interest.

Australian Foreign Minister Kevin Rudd vowed the country's diplomats would support Assange, even after the whistleblower accused Canberra of "disgraceful pandering" to his foes.