Jailed Australian journalist Peter Greste has described his ordeal on trial in Egypt as a "massive injustice", after spending more than four months in jail.

Greste is on trial with 19 co-defendants, including five Al Jazeera journalists, for allegedly defaming the country and having ties to the blacklisted Muslim Brotherhood.

Saturday's hearing in Cairo, where Greste's bail was again refused, coincided with World Press Freedom Day.

"We recognise the significance of the coincidence of this trial falling on World Press Freedom Day. This is a very clear message," Greste said from his caged dock at the court in Cairo.

Greste, a Peabody Award-winning journalist who previously worked with the BBC, also branded the trial "a massive injustice, regardless of the outcome".

Most of the defendants are being tried in absentia.

The Al Jazeera journalists in the dock along with Greste are the broadcaster's English channel Cairo bureau chief Mohamed Fahmy and producer Baher Mohamed.

The three journalists were arrested in Egypt four months ago and accused of broadcasting false news and aiding the outlawed Muslim Brotherhood.

Their trial comes against the backdrop of strained relations between Cairo and Qatar, which supports the Muslim Brotherhood movement of deposed Islamist president Mohammed Morsi.

It has sparked an international outcry and calls for the release of the journalists, while Cairo insists the trial does not presage a wider crackdown on journalists.

In a letter from prison published on Friday, Fahmy said he was on trial for simply doing his job.

"I write you this letter on World Press Freedom Day from my cell after 126 days of incarceration for doing nothing more than the job I love," he wrote.

The judge on Saturday adjourned the trial to May 15.

Greste says campaign for release has sent 'unequivocal response'

Greste's parents, Lois and Juris, read out a statement from their son on Friday ahead of his court appearance.

"If by arresting us, the (Egyptian) government sought to send a message to journalists both foreign and local working here, then the campaign for our release has sent an unequivocal response," the statement said.

"It is one that is being heard everywhere.

"We are confident that in time and hopefully soon all three of us will be acquitted and released."

Greste's parents said they would keep fighting for his release.

"We are not leaving any stone, any little pebble unturned because as parents we just cannot rest until Peter is out and exonerated," Mr Greste said.

AFP