The full extradition hearing of WikiLeaks founder Julian Assange will go ahead in February 2020 after a London judge declined a request by his lawyers to delay proceedings by three months.

Key points: Julian Assange was denied a request to delay his extradition proceedings by three months, with the full hearing still set to take place over five days in late February 2020

Julian Assange was denied a request to delay his extradition proceedings by three months, with the full hearing still set to take place over five days in late February 2020 Assange's lawyer Mark Summers accused the US of illegally spying on Assange while he was inside the Ecuadorian Embassy seeking refuge

Assange's lawyer Mark Summers accused the US of illegally spying on Assange while he was inside the Ecuadorian Embassy seeking refuge Scores of the WikiLeaks founder's supporters filled the courtroom and protested outside it

The 48-year-old appeared in a packed court on Monday to fight extradition to the United States, where he faces 18 counts, including conspiring to hack into Pentagon computers and violating an espionage law.

Britain's former Home Secretary Sajid Javid signed an order in June allowing Assange to be extradited to the US, where authorities accuse him of scheming with former Army intelligence analyst Chelsea Manning to break a password for a classified government computer.

He could spend decades in prison if convicted.

Scores of Julian Assange's supporters filled the public gallery of the courtroom and held up placards. ( AP: Kirsty Wigglesworth )

Assange and his legal team said he needed more time to prepare his case, but failed to convince District Judge Vanessa Baraitser that a slowdown was justified.

The full extradition is still set for a five-day hearing in late February, with brief interim hearings in November and December.

Assange — clean shaven, with his silvery-grey hair slicked back — defiantly raised a fist to supporters who jammed the public gallery in Westminster Magistrates Court.

After the judge turned down his bid for a three-month delay, Assange, speaking very softly and at times appearing to be near tears, said he did not understand the proceedings.

He said the case was not "equitable" because the US government had "unlimited resources" while he did not have easy access to his lawyers or to documents needed to prepare his battle against extradition while confined to Belmarsh Prison on the outskirts of London.

"They have all the advantages," Assange — who argues he is a journalist entitled to First Amendment protection — said.

Lawyer Mark Summers, representing Assange, told the judge that more time was needed to prepare Assange's defence against "unprecedented" use of espionage charges against a journalist.

Mr Summers said the case has many facets and would require a "mammoth" amount of planning and preparation.

He also accused the US of illegally spying on Assange while he was inside the Ecuadorian Embassy seeking refuge, and of taking other illegal actions against the WikiLeaks founder.

"We need more time," Mr Summers said, adding that Assange would mount a political defence.

Mr Summers said the initial case against Assange was prepared during the administration of former president Barack Obama in 2010 but wasn't acted on until Donald Trump assumed the presidency.

Assange faces 18 counts in the US including violating espionage laws, and could spend decades in prison if convicted. ( AP: Kirsty Wigglesworth )

He said it represented the US administration's aggressive attitude toward whistleblowers.

Representing the US, lawyer James Lewis opposed any delay to the proceeding.

The case is expected to take months to resolve, with each side able to make several appeals of rulings.

The judge said the full hearing would be heard over five days at Belmarsh Court, which would make it easier for Assange to attend and contains more room for the media.

Assange's lawyers said the five days would not be enough for the entire case to be heard.

Assange defiantly raised a fist to his supporters in the courtroom, while others outside there were chants calling for him to be set free. ( AP: Kirsty Wigglesworth )

Health concerns for Assange

Outside the courthouse, scores of his defenders — including former London mayor Ken Livingstone — carried placards calling for Assange to be released.

Wikileaks editor-in-chief Kristinn Hrafnsson said it was a "big test case for journalism worldwide".

"This should be thrown out immediately because this is a total violation of a bilateral treaty between the US and the United Kingdom which basically states that you cannot extradite someone for political offences, and this is a political case," he said.

Regarding Assange's health, Mr Hrafnsson said he was in a "stable condition" but was living in "de facto solitary confinement".

"After three or four weeks it starts to bite in and you can feel that he is suffering," he said.

"It is torture, and he stated it today here himself."

Assange supporter Malcolm, who did not give his surname, told the ABC there was "not nearly enough" people actively campaigning for Assange's freedom, and he wanted to see the whole street blocked at the next hearing.

Another supporter accused the Australian government of failing to "defend their own citizen".

The crowd outside court was largely well-behaved but briefly blocked traffic when a prison van believed to be carrying Assange left court.

ABC/wires