A full-page advertisement in Monday's Guardian newspaper has demanded the immediate release of BBC correspondent Alan Johnston, who was abducted in Gaza three weeks ago. Over 300 journalists and media personalities - including the director-general of the BBC, many national newspaper editors and the editor of the Al-Jazeera TV network - have signed the open letter urging "everyone with influence" to increase their efforts to ensure Johnston is "freed quickly and unharmed." Meanwhile, protests and vigils continue to take place in Gaza. On Sunday, foreign journalists stood in protest with their mouths gagged, as Palestinian journalists began a boycott of the Palestinian Authority and presidency, claiming they were not doing enough to ensure Johnston's release. Naim Toubasi, head of the Palestinian Journalists Syndicate, told the BBC it was necessary to boycott Palestinian government proceedings because the authorities were doing little to help secure Johnston's release. In response, Palestinian media outlets would not cover stories on the activities of the presidency or the new unity government until Johnston was freed, he said. Majed Said, an Abu Dhabi television correspondent, told the BBC: "This is unacceptable for us... we will boycott coverage of President [Mahmoud] Abbas if Johnston is not released." Johnston has been the BBC's Gaza correspondent for three years; he was kidnapped on March 12. To date, there has been no word on his whereabouts or condition, and no group has claimed responsibility or demands made. The Guardian advertisement says that Johnston decided to live in Gaza as he felt it was the only way to tell the story of the people. The Arab League and the European Union, as well as various charities and non-government organizations, have made appeals for Johnston's release, and senior BBC colleagues have asked Palestinian leaders, including Hamas Prime Minister Ismail Haniyeh, for their help.