Stella Assange told the PA news agency she is concerned for his health, which has deteriorated since he was sent to Belmarsh prison three years ago after being dragged out of the Ecuadorian embassy in London.

He is embroiled in a lengthy legal battle to avoid extradition to the United States.

News of the COVID-19 infection emerged over the weekend as thousands of people staged demonstrations in London, Washington DC and Melbourne to demand Assange's release from prison.

The positive test came out on Saturday, Stella Assange said.

"Julian was feeling unwell last week but started feeling sick on Friday. He started coughing and had a fever," she said.

"He was given some paracetamol. He tested positive for COVID on Saturday, the same day thousands of people came out onto the streets to support him."

Stella Assange said she was concerned about his health.

"I am obviously worried about him and the next few days will be crucial for his general health. He is now locked in his cell for 24 hours a day," she said.

Stella Assange said she was overjoyed at the number of people who formed a human chain around parliament on Saturday, estimating there were well over 5000 in attendance.

It was the biggest event of its kind in support of the WikiLeaks founder, who has won support from human rights organisations, journalist groups and others across the world.

Last month, US lawyers and journalists who visited Assange when he was at the Ecuadorian embassy said they are suing the CIA, claiming it spied on their private conversations in violation of the Fourth Amendment.

The UK Ministry of Justice has been approached for a comment.

Australian Associated Press