(Adds details, Human Rights Watch comment)
By Seham el-Oraby
CAIRO, Feb 20 (Reuters) - Egypt put three Al Jazeera journalists on trial on Thursday on charges of aiding members of a "terrorist organisation", in a case that human rights groups say shows the authorities are trampling on freedom of expression.
The journalists, wearing white prison outfits, appeared in metal cages, a Reuters witness said. Six others identified as Al Jazeera journalists are being tried in absentia.
Three of the Qatar-based television network's journalists - Peter Greste, an Australian, Mohamed Fahmy, a Canadian-Egyptian national, and Baher Mohamed, an Egyptian - were detained in Cairo on Dec. 29 and remain in custody, Al Jazeera said.
All three deny the charges and Al Jazeera has said the accusations are absurd. Egyptian officials have said the case is not linked to freedom of expression and that the journalists raised suspicions by operating without proper accreditation.
"Journalists should not have to risk years in an Egyptian prison for doing their job," said Joe Stork, deputy Middle East director at Human Rights Watch.
"The prosecution of these journalists for speaking with Muslim Brotherhood members, coming after the prosecution of protesters and academics, shows how fast the space for dissent in Egypt is evaporating."
The court postponed the case, in which a total of 20 people are on trial, to March 5.
"We believe we will be acquitted. The lawyers are fully on board with us, they fully believe in our case, they fully believe that we were just operating as journalists," said Heather Alan, head of news gathering at Al Jazeera English.
"We don't have an agenda, we have nothing against Egypt, we certainly don't lie or do biased reporting. So we believe that we are innocent," she said, speaking outside the court.
Last month a prosecutor said Al Jazeera journalists had published lies harming the national interest and had supplied money, equipment and information to 16 Egyptians. The foreigners were also accused of using unlicensed broadcasting equipment.
The 16 Egyptians are to face trial for belonging to a "terrorist organisation", an apparent reference to the Muslim Brotherhood, which has been protesting against the government since the army toppled Islamist President Mohamed Mursi in July.
"I have been subjected to torture," said Suhaib Saed, who is accused of being a member of a terrorist group.
The government has declared the Brotherhood a terrorist group. The Brotherhood says it is a peaceful organisation.
Andrew Greste said his award-winning brother Peter had only been in Cairo on a three-week assignment to replace a colleague.
"So it's completely, from my point of view, unbelievable to have those accusations levelled against him," he told Reuters outside the court.
"The most difficult thing for him is the mental challenge of staying positive and focused and not allowing the conditions that he's living under to become overwhelming and depressing."
The Gulf state of Qatar, which funds Al Jazeera, backs the Muslim Brotherhood. Its ties with Egypt have been strained since army chief Field Marshal Abdel Fattah al-Sisi ousted Mursi last year after mass protests against his troubled one-year rule.
Both state and private Egyptian media have whipped up anti-Brotherhood sentiment, suggesting anyone associated with the group is a traitor and a threat to national security.
Egyptians often ask journalists in the streets whether they work for Al Jazeera. Saying yes could mean a beating.
The crackdown on dissent has raised questions about Egypt's democratic credentials three years after an uprising toppled autocrat Hosni Mubarak and raised hopes of greater freedoms. (Reporting by Omar Fahmy; Writing by Michael Georgy; Editing by Alistair Lyon)