Visa says it has suspended all payments to WikiLeaks pending an investigation of the organization's business.
Visa's decision is a powerful blow to the loosely knit organization, which relies on online donations to fund its operations.
Popular online payment company PayPal, Inc. has already severed its links with WikiLeaks. Visa's decision to pull the plug on WikiLeaks leaves the website with one fewer source of revenue.
Swiss authorities closed Assange's new Swiss bank account Monday.
THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.
LONDON (AP) -- WikiLeaks founder Julian Assange surrendered to London police Tuesday to face a Swedish arrest warrant, the latest blow to an organization that faces legal, financial and technological challenges after releasing hundreds of secret U.S. diplomatic cables.
Assange was at Westminster Magistrate's Court on Tuesday afternoon, waiting to attend a hearing. His Swedish lawyer told The Associated Press his client would challenge any extradition from Britain to Sweden.
If that is the case, Assange will likely be remanded into U.K. custody or released on bail until another judge rules on whether to extradite him, a spokeswoman for the extradition department said on customary condition of anonymity.
Assange, a 39-year-old Australian, has been accused by two women in Sweden. He faces rape and sexual molestation allegations in one case and sexual molestation and unlawful coercion in the other. Assange denies the allegations.
His British attorney Mark Stephens says the allegations stem from a "dispute over consensual but unprotected sex" last summer.
Swedish prosecutor Marianne Ny has rejected claims by Stephens and Assange that the prosecution has political overtones. She planned to comment on the arrest later Tuesday.
Assange's Swedish lawyer Bjorn Hurtig said his client would contest any extradition.
"He will absolutely do that," he told the AP in a telephone interview.
Hurtig said it was difficult to say how long the extradition process in Britain would take -- anywhere from a week to two months. He said if Assange was extradited to Sweden, he wouldn't be kept in detention after he's been questioned, "because it's been for the sake of the questioning that he's been detained."
U.S. Defense Secretary Robert Gates, visiting with Afghan President Hamid Karzai and U.S. troops in Afghanistan, was pleased by the arrest.
"That sounds like good news to me," he said.
Beginning in July, WikiLeaks angered the U.S. government by releasing tens of thousands of secret U.S. military documents on the wars in Afghanistan and Iraq. That was followed last week by the ongoing release of what WikiLeaks says will eventually be a quarter-million cables from U.S. diplomatic posts around the world. The group provided those documents to five major newspapers, which have been working with WikiLeaks to edit the cables for publication.
In the past week, WikiLeaks has seen its bank accounts canceled and its web sites attacked. The U.S. government has launched a criminal investigation, saying the group has jeopardized U.S. national security and diplomatic efforts around the world.
WikiLeaks has also seen an online army of supporters come to its aid, sending donations, fighting off computer attacks and setting up over 500 mirror sites around the world to make sure that the secret documents are published regardless of what happens to Assange.
A spokesman for WikiLeaks called Assange's arrest an attack on media freedom and said it won't prevent the organization from releasing more secret documents.
"This will not change our operation," Kristinn Hrafnsson told The Associated Press.
But Hrafnsson also said the group had no plans at the moment to release the key to a heavily encrypted version of some of its most important documents -- an "insurance" file that has been distributed to supporters in case of an emergency. Hrafnsson said that will only come into play if "grave matters" involving WikiLeaks staff occur -- but did not elaborate on what those would be.
The campaign against WikiLeaks began with an effort to jam the website as the cables were being released. U.S. Internet companies Amazon.com, Inc., EveryDNS and PayPal, Inc. then severed their links with WikiLeaks in quick succession, forcing it to jump to new servers and adopt a new primary Web address -- wikileaks.ch -- in Switzerland.
Swiss authorities closed Assange's new Swiss bank account Monday, and MasterCard has pulled the plug on payments to WikiLeaks, according to technology news website CNET.
The attacks appeared to have been at least partially successful in stanching the flow of secrets: WikiLeaks has not published any new cables in more than 24 hours, although stories about them have continued to appear in The New York Times and Britain's The Guardian, two of the newspapers given advance access to the cables.
WikiLeaks' Twitter feed, generally packed with updates, appeals and pithy comments, has been silent since Monday night, when the group warned that Assange's arrest was imminent.
Louise Nordstrom in Stockholm and Greg Katz and Cassandra Vinograd in London contributed to this story.