The 39-year-old Australian was brought into court in a prison van before the hearing as supporters and journalists gathered in the street and in the courtroom.
the founder of WikiLeaks, has reaffirmed his committment to publishing secret US cables, while denouncing companies that suspended payments to his website, Australian television reported.
Australia's Network Seven said on Tuesday that Assange made the comments from his prison cell in London during a 10-minute phone conversation with his mother, Christine Assange.
The news came ahead of a London court hearing at which Assange's lawyers will attempt to win his release on bail.
"My convictions are unfaltering. I remain true to the ideals I have expressed," Assange said, according to his mother who supplied Network Seven with a written statement from her son.
"This circumstance shall not shake them. If anything, this process has increased my determination that they are true and correct," he wrote.
Provoking fury
WikiLeaks has provoked fury in Washington with its publications of secret US cables and has vowed to make public the 250,000 embassy documents it has obtained.
On Tuesday, Assange was also critical of the major finance companies who suspended payments to his WikiLeaks site.
"We now know that Visa, Mastercard, Paypal and others are instruments of US foreign policy. It's not something we knew before," he said, and called for the protection of his work and his team from "illegal and immoral attacks".
Internet activists launched "Operation Payback" to avenge WikiLeaks against those perceived to have obstructed its operations.
They temporarily brought down the websites of credit card firms Visa and MasterCard, as well as that of the Swedish government, last week.
Assange handed himself in to British police last week after Sweden issued a European warrant for his arrest for sex crime charges. "He's been held in the basement of the prison, right in the middle of the prison in solitary confinement," his mother said.
Christine Assange told her son that there was worldwide support for his cause. "As a mother I am asking the world to stand up for my brave son," she said.
Assange was accused this year of sexual misconduct by two female Swedish WikiLeaks volunteers during a stay in Sweden. He denies the allegations.
The WikiLeaks founder and his lawyers have voiced fears that US prosecutors may be preparing to indict him for espionage after the WikiLeaks website published secret US documents.
The City of Westminster Magistrates' court was packed with lawyers, journalists and celebrities, including filmmaker Ken Loach and socialite Jemima Khan who have both offered to contribute 20,000 pounds ($31,500) each in bond for Assange.
Assange is wanted for questioning after two women accused him of sexual misconduct in separate encounters in Sweden over the summer. Lawyers for Assange say he denies the allegations and will contest Sweden's attempt to extradite him for questioning.
Assange was placed into custody at a hearing a week ago after surrendering to Scotland Yard to answer a Swedish arrest warrant.
Assange's lawyer, Mark Stephens, said his client would offer to be electronically tagged and stay at an address known to the police.
"One's never going to count one's chickens until they're hatched, but I hope that in these circumstances the district judge will feel confident" granting bail, Stephens told Sky News.
