He was the only western journalist to live and work in Gaza and was only two weeks away from the end of a three-year stint there when he was abducted.
Despite repeated appeals for Mr Johnston's release – from his family, the BBC and the UK and Palestinian governments – no information has yet been received about his whereabouts.
"This is a desperately worrying time for us. We make a heartfelt appeal to anybody who may have knowledge of Alan's situation and wellbeing to contact the authorities in Gaza," Mr Johnston's family said.
BBC director general Mark Thompson commented: "Alan Johnston is a remarkable, courageous journalist who stayed in Gaza because he believes the story of Gaza needs to be told.
"Seven weeks after he was abducted, we want Alan home, safe and well."
The BBC's rally at the Ralph Bunche park opposite the UN plaza in New York is set to begin at 14:00 local time (19:00 BST) and will be led by the organisation's world editor, Jon Williams. It takes place on world press freedom day, 52 days after Mr Johnston's abduction.
Foreign office minister Ian McCartney said this year's theme of press freedom, safety of journalists and impunity "comes as a timely reminder of the threats faced by journalists".
"I would like to take this opportunity to reaffirm our commitment to promote, defend, and protect the right of journalists and media workers to work without fear of reprisal," he commented.
"Sadly, as we have seen all too often, journalists are targeted for the work they do."
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