By Mark Trevelyan, Security Correspondent

LONDON, Sept 8 (Reuters) - Abandoning his Kalashnikov and dyeing his beard from grey to black, Osama bin Laden presents a new image to the world in a video that makes no specific threats but may be a signal for new al Qaeda attacks.

In a half-hour address released four days before the sixth anniversary of the Sept. 11 attacks on the United States, bin Laden lurched between history lesson and sermon, urging Americans to ditch capitalist democracy and embrace Islam if they want to end the war in Iraq.

Security analysts said the long-awaited video -- bin Laden's first for nearly three years -- disproved rumours of his death but was mainly significant for its style rather than its content.

"He appeared without the military camouflage jacket he used to wear, without his favourite Kalashnikov which he captured from a Soviet general during the Afghan war," said Abdel Bari Atwan, London-based editor-in-chief of the Arabic newspaper al-Quds.

By dyeing his hair and beard and wearing Arabic robes, bin Laden was trying to portray himself as a new, mature figure -- the spiritual leader of al Qaeda, Atwan said.

Others said the makeover was bizarre.

"It makes him, a man who claims he wants to be a martyr, look vain and ridiculous," said M.J. Gohel of the Asia-Pacific Foundation, a London think-tank.

Despite the lack of dramatic content, analysts said they could not rule out the possibility that bin Laden's first appearance since the eve of the 2004 U.S. presidential election was a rallying cry for a new attack.

"Because video tapes featuring bin Laden speaking to camera are so rare, the release of this particular tape could herald a major attack, though remarkably this message contains none of his usual open threats against the United States," Gohel said.

Atwan, who has interviewed bin Laden, told Reuters: "Maybe this is a warning that an attack could happen soon...This is a sort of rallying video. Maybe there is a message to his followers: go ahead and do what you want to do."

LONG SILENCE

Bin Laden did not explain his long silence, which had prompted speculation he was too sick or too tighly holed up in a hiding place somewhere along the Pakistan-Afghan border to be able to make and smuggle out a message.

Amr El-Choubaki, a Cairo-based expert on Islamist movements, said the call for the U.S. to convert to Islam was a sign he was not in a position to name more achievable objectives.

"It's clear his influence within the al Qaeda organisation... is now limited," he said.

But Mohamed el-Sayed Said, deputy director of the Ahram Centre for Political and Strategic Studies in Cairo, said the video, despite the lack of specific warnings, was "much more threatening this time".

"It's confident, it uses iconic language that suggests, 'I'm commissioned to wage an unending war against you, and the only way to get peace is to convert to Islam'," he said.

"He's in a state of battle, a state of constant, unending war until he Islamises the world."

The address contained no tactical gambit of the type attempted previously by bin Laden, when he tested the resolve of Western governments by offering them a "ceasefire" if they withdraw troops from Muslim countries.

Instead he ranged across religion, history, domestic U.S. politics and the wars in Iraq and Afghanistan, throwing in climate change and even what appeared to be a reference to the current crisis over bad mortgage loans in the United States.

Arab security analysts said bin Laden's attempt to restyle himself as civilian leader and ideologist underlined the evolution of al Qaeda from a centralised system to a loose, horizontal one in which operations are led by local commanders.

"This is just a message to his followers and foes that he exists and that he is still the leader of al Qaeda," said Saudi analyst Fares bin Houzam.

"I am 100 percent sure that this man has no power to plan (for al Qaeda). He is just giving signals to his followers around the world." (Additional reporting by Inal Ersan in Dubai and Aziz El-Kaissouni in Cairo)

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