Freed WikiLeaks founder Julian Assange said he expected further "smears" and a potential indictment from the US on his first day under house arrest.

The 39-year-old whistleblower was let out of prison after a judge ruled he should be released ahead of Swedish extradition proceedings in the new year.

He vowed to "continue his work and protest his innocence" after emerging from the High Court to face the world press after nine days behind bars.

Speaking at the East Anglian mansion at which he has been ordered to stay, the Australian said he expected further leaked information relating to claims of sexual assault against him to be made public some time today.

He also indicated that the US is preparing to indict him on espionage charges.

"We have heard from one of my US lawyers, yet to be confirmed, but a serious matter, that there may be a US indictment for espionage for me, coming from a secret US grand jury investigation," he said.

A spokeswoman from the US Department of Justice would only confirm that there is "an ongoing investigation into the WikiLeaks matter".

Assange is wanted in Sweden for alleged sex offences which he denies. His lawyers have accused the Swedish authorities of waging a "vendetta" against him.