Some 300 journalists on Monday called for the release of British Broadcasting Corp. correspondent Alan Johnston, abducted in the Gaza Strip three weeks ago. The appeal came in a full-page advertisement in The Guardian newspaper signed by many of Britain's best-known journalists, including veteran interviewer David Frost, host David Dimbleby and BBC presenter Jeremy Paxman, and by the editors of several national newspapers. Johnston, 44, was kidnapped from his car by masked gunmen on March 12. He had lived in Gaza for three years, the only Western reporter permanently based in the violence-wracked Palestinian territory. Several other Western journalists have been abducted in Gaza; none was held as long as Johnston and all were released unharmed. Many suspect that members of a powerful Gaza clan took the BBC reporter, but moves to free him have so far made little progress.