Journalists, politicians and human rights activists will gather outside the Egyptian embassy in London today to demand an end to intimidation of the media in the Middle East country.
They will call for the immediate release of all detained journalists, a day before the trial of four of them begins.
The demonstration, organised by the National Union of Journalists (NUJ), comes a day after major news organisations wrote to the Egyptian authorities protesting against the continuing imprisonment of a number of reporters.
BBC News, ITN, Sky, Reuters, NBC News and ABC News called for them to be set free.
They include correspondent Peter Greste, who was arrested with two colleagues working for Al Jazeera in a Cairo hotel on December 29.
Twenty journalists working for the network face charges of joining or aiding a terrorist group and endangering national security.
The letter says: "Since December 29 last year the foreign correspondent Peter Greste has been held with two Al Jazeera colleagues in Egypt's Tora prison, accused of news reporting which is 'damaging to national security'. This week he goes on trial.
"We know Peter Greste to be a fine, upstanding correspondent who has proved his impartiality over many years, whichever of our organisations he has been working for, and in whichever country."
The letter tells how Greste left his native Australia in 1991 to work as a freelancer for Reuters TV, CNN, WTN and the BBC. In 1995 he was the correspondent in the Afghan capital Kabul for the BBC and Reuters. He then worked for the BBC News channel in London, and as a BBC foreign correspondent in Mexico and Chile.
It adds: "He returned to Afghanistan to cover the start of the war in 2001, then roamed across the Middle East, Latin America and, for the last nine years, Africa, where he is the correspondent for Al Jazeera. In all that time, he has polished his journalistic reputation."
The letter said Greste was being put on trial because of the Egyptian government's decision on December 25 to add the Muslim Brotherhood to its list of terrorist organisations.
Greste had written that when this happened "it knocked the middle ground out of the discourse. When the other side, political or otherwise, is a 'terrorist', there is no neutral way... So, even talking to them becomes an act of treason, let alone broadcasting their news, however benign."
Under the charges, foreign-born journalists could face up to seven years in jail, and Egyptian journalists up to 15 years.
Journalists from Al Jazeera English under detention are Greste, Mohammed Fadel Fahmy and Baher Mohammed. Their colleague Abdullah Al Shami, of Al Jazeera Arabic, has been detained since August 14 last year and is in the third week of a hunger strike, the NUJ said.
Union general secretary Michelle Stanistreet said: "The situation facing journalists working in Egypt is dire. The arrests and brutal attacks are a deliberate attempt to silence journalists and prevent them from doing their job - giving citizens access to vital information and news.
"The trial of Al Jazeera English journalists, on ludicrous allegations of damaging national security, should be halted and all journalists immediately released."
The Egyptian authorities published a list on February 5 of 20 journalists they accused of aiding terrorists while working in the country. Of the 20, nine are Al Jazeera staff.
Correspondent Sue Turton, one of the 20 accused, is expected to attend the demonstration.
Ms Turton, an award-winning correspondent who worked for Sky News, ITN and Channel 4 before Al Jazeera, and who has reported from Afghanistan, Libya and Ukraine, said: "I am astounded that a warrant is out for my arrest because of my reporting in Egypt last year.
"I didn't treat the situation there any differently to every other story I've reported on in almost 25 years as a TV reporter. I have no allegiance to any political group in Egypt or anywhere else and no desire to promote any one point of view."