UNITED NATIONS, May 3 (Reuters) - Calling for the release of a BBC correspondent held in Gaza, the U.N. deputy secretary-general told a rally outside U.N. headquarters on Thursday that no cause was served by his captivity.

Among the panels and luncheons to commemorate World Press Freedom Day was a British Broadcasting Corp.-organized outdoor rally attended by dozens of journalists and U.N. staff demanding freedom for Alan Johnston, abducted in the Gaza Strip 52 days ago.

"No cause is served by Alan's captivity and any cause is undermined by Alan's captivity," said Rose-Ashe Migiro, the Tanzanian deputy secretary-general, filling in for Secretary-General Ban Ki-moon, who is in Sharm el-Sheikh, Egypt, addressing conferences on Iraq.

Johnston, 44, the only foreign correspondent based full-time in Gaza, has not been heard of since his car was found abandoned on March 12. The Palestinian government talked with his abductors on Monday but rejected ransom demands,

The American ABC network came out in force, including former news anchor Bob Woodruff, who was nearly killed by a roadside bomb in Iraq. Martin Bashir, co-anchor of "Nightline," told the gathering that Johnston's "only concern has been to tell the truth and to do so with humanity and compassion."

Ban, in a statement, also appealed for Johnston's immediate release. So did Sheikha Haya Rashed Al Khalifa of Bahrain, the president of the U.N. General Assembly, who said that "not just Alan Johnston, every reporter unlawfully imprisoned must be set free."

Some 134 journalists were imprisoned worldwide as of December and 56 were killed in 2006 alone, according to the New York-based Committee to Protect Journalists.

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