UN Secretary General Ban Ki-moon has added his voice to calls for the release of BBC correspondent Alan Johnston, abducted in Gaza a month ago.
He said freedom of the press should be protected "as a matter of principle".
On an international day of action to highlight the veteran reporter's plight, his parents also made a direct appeal to his captors for his release.
The BBC, al-Jazeera, and Sky News simulcast a special live programme on the dangers facing journalists in Gaza.
Speaking in New York, Mr Ban said: "I sincerely hope that those who are responsible for this abduction should release him unconditionally and immediately.
"I will do whatever I can in my capacity as secretary-general but my sympathy is with the family of Mr Alan Johnston."
Head of news for BBC World, Richard Porter, said the BBC had wanted to create a programme which had impact in the Middle East and around the globe.
A BBC poster campaign has also been unveiled at prominent sites in London and Mr Johnston's native Scotland.
Alan's father, Graham Johnston, read a message to his son, telling him not to worry about his family, saying that they were "hanging in there" and "keeping a stiff upper lip".
Mr Johnston thanked the BBC, his son's fellow journalists and the Palestinian people for their support.
"We are overwhelmed at such support, all of which helps to buoy us up during the past seemingly interminable four weeks," he said at the news conference in London.
He appealed directly to the kidnappers, saying: "You have family. Please think about what this is doing to my family. Please let my son go now, today."
At Trafalgar Square in central London, the media rights group Reporters Without Borders staged a rally to appeal for Mr Johnston's freedom.
In Gaza City, about 200 Palestinian journalists gathered at government and security offices to demand more action for the release of their colleague.
To date, 12,000 people have signed a BBC online petition demanding the immediate release of Mr Johnston.
In the West Bank, BBC director general Mark Thompson also made a direct appeal for his release, saying there were growing concerns for his well-being.
Mr Thompson met Palestinian Authority President Mahmoud Abbas on Wednesday.
"He told me that he had credible evidence that Alan was safe and well. He assured me that Palestinian Authority is fully engaged with Alan's case and working to resolve this as soon as possible," Mr Thompson said.
But he emphasised that there had been no contact with any kidnappers and no demands had been received.
He went on: "It is vital for all journalists to be able to report freely and without fear of harassment and intimidation. The people of Gaza are ill-served by kidnappings of this nature."
A half-hour programme was broadcast from the West Bank town of Ramallah, anchored by Jeremy Bowen, the BBC's Middle East Editor. It included reports from al-Jazeera, CNN and Sky.
Introducing the programme, Mr Bowen said: "About the only good thing to come out of the last month is the way Alan's colleagues, especially here in the occupied Palestinian territories, have rallied around him.
"This is the first time that global news networks have come together like this. We stand united in support of Alan Johnston - and all our colleagues who are in harm's way," he said.
The chairman of the Foreign Press Association, Simon McGregor-Wood, told the programme that news coverage of Gaza would suffer as a result of Mr Johnston's disappearance.
"I think it is the stories that provide context and background, very important for our audience to understand Gaza, those are the stories that are simply not going to be done.
"It is too dangerous, people are scared," he said.
Alan Johnston was taken hostage by masked gunmen as he returned to his apartment in Gaza City on 12 March.
Kidnappers have abducted dozens of foreigners in Gaza, but none have been held so long as Mr Johnston.
The 44-year-old 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.
He has lived and worked in Gaza for three years and was the only Western reporter permanently based in the often violent and lawless territory.
His posting in Gaza had been due to end in late March.