Protests and vigils are continuing for BBC correspondent Alan Johnston, three weeks to the day after his suspected abduction in the Gaza Strip.
The Guardian advert features names from the BBC and outside
Palestinian journalists began a boycott of their government and presidency to get them to do more for his release.
Events were held in the West Bank and London. In Gaza, 300 journalists stood with their mouths gagged.
Dozens of media personalities signed an open letter in a UK newspaper urging him to be freed quickly and unharmed.
David Dimbleby, Sir David Frost, Jon Snow and Christiane Amanpour of CNN are among hundreds of Mr Johnston's friends and colleagues who backed a full-page advert in Monday's Guardian newspaper.
"It could have been any of us," Channel 4 News presenter Jon Snow told the BBC. "We have to stand together against this."
Other signatories include:
- BBC Director-General, Mark Thompson
- Director of channels at Channel 4, Kevin Lygo
- Editors of several London newspapers
- Wadah Khanfar, editor-in-chief of Arabic TV network al-Jazeera
The BBC has had no contact since 12 March with Mr Johnston, who has lived and worked in Gaza for the past three years.
He is widely believed to have been kidnapped, but no demands have been made public.
The head of the Palestinian Journalists' Union, Naim Tubasi, told the BBC it was necessary to boycott Palestinian government proceedings because the authorities were doing little to help secure the journalist's release.
"This is unacceptable for us... We will boycott coverage of (President Mahmoud) Abbas if Johnston is not released," said Abu Dhabi TV correspondent Majed Said.
During the boycott, Mr Tubasi said no Palestinian media outlet would cover stories on the activities of the presidency or the new unity government. He said the protests would continue until Mr Johnston was freed.
Intensive international efforts have been going on to secure his safe return.
Palestinian journalists are stepping up action for Alan Johnston
These include appeals from the Arab League and the European Union and non-government groups such as Amnesty International and Reporters Without Frontiers.
The International Press Institute, a media watchdog, expressed "grave concern" about Alan Johnston's fate.
IPI director Johann Fritz called on anyone with influence in Gaza to intensify their efforts to ensure his freedom.
"His work reporting fairly and accurately from Gaza has been widely praised. He has shed light on one of the world's most demanding stories - in keeping with the highest standards of the BBC," Mr Fritz said in a statement.
Senior BBC colleagues of Mr Johnston have appealed to Palestinian leaders including Mr Abbas and Palestinian Prime Minister Ismail Haniya.
Both have said everything possible is being done to free Mr Johnston.
The BBC describes him as a highly experienced and respected reporter.
He joined the BBC World Service in 1991 and has spent eight of the last 16 years as a correspondent, including periods in Uzbekistan and Afghanistan.