Palestinian journalists in vehicles hold posters of BBC correspondent Alan Johnston during a demonstration calling for his release in Gaza City. There has been no sign of Johnston since he was abducted at gunpoint March 12, and no word from his captors. Three international news networks called for Johnston's release during a joint broadcast Thursday.

Ramallah, West Bank ? Three international news networks had a joint broadcast Thursday calling for the release of BBC journalist Alan Johnston a month after he was kidnapped by Palestinian gunmen in Gaza City.

The 25-minute broadcast by the British Broadcasting Corp., Al-Jazeera and Sky News – with a contribution from CNN – was part of a day of public events meant to put pressure on the kidnappers, including an appeal from Johnston’s father to “let my son go. Now. Today!”

Alan Johnston, a Scotland native who was abducted at gunpoint March 12, has been held longer than any other foreigner kidnapped in Gaza. Johnston, the only foreign reporter based in Gaza, was snatched just weeks before he was scheduled to end his three-year stint there. There has been no sign from him since, and no word from his captors.

Palestinian President Mahmoud Abbas told the BBC on Wednesday he had “credible evidence that Alan was safe and well,” BBC director general Mark Thompson said during a news conference Thursday in Ramallah.

Abbas aide Saeb Erekat said the Palestinian president had been reassured that Johnston was alive, and Abbas “is exerting every possible effort to ensure his release.”

“I have never been more ashamed as a Palestinian than what I feel now with the continued abduction of Johnston,” Erekat said.

The so-called “Day of Action” on behalf of Johnston was organized by the BBC to ensure that his plight is not forgotten.

In a morning news conference in London, the journalist’s father, Graham Johnston, read an appeal to those holding him.

“You have families. Please think about what this is doing to my family,” he said.

He also told his son not to worry about his family and to maintain hope.

“Chin up, my son,” he said.

In New York, U.N. Secretary-General Ban Ki-moon said Thursday he was “deeply concerned” that Johnston still was being held captive and pledged to do whatever he could to secure his release.

“I sincerely hope that those who are responsible for this abduction should release him unconditionally and immediately,” he said at U.N. headquarters.

More than a dozen foreign journalists and aid workers have been abducted by gunmen in Gaza in the past 18 months, often in a bid by Palestinian militants to get money or jobs. Most have been released unharmed within hours or days. The one exception was the abduction of two Fox News employees in August. They were held for two weeks before they were freed, unharmed.