The astonishing terrorist strikes in the United States quickly reached a global audience, with many around the world watching live coverage of an aircraft hitting the World Trade Centre.

Audiences were transfixed by the awful images from New York, where one World Trade Centre tower collapsed.

Russian President Vladimir Putin expressed his condolences to the American people over the terrorist attacks, calling the "terrible tragedies," the Kremlin press service said.

"I'm afraid we can only imagine the terror," said Prime Minister Tony Blair, who cancelled a scheduled speech at a trade union conference.

"I know that you would want to join with me in sending the deepest condolences to President Bush and the American people on behalf of the British people."

"This mass terrorism is the new evil in our world today," Blair continued. "It is perpetrated by fanatics who are utterly indifferent to the sanctity of human life, and we the democracies of this world are going to have to come together and fight it together."

In Berlin, Foreign Ministry officials in Berlin huddled in a crisis meeting, and Parliament's vice speaker vice speaker Anke Fuchs told MPs a "terrible catastrophe" had happened.

Virtually all German TV channels switched to live coverage. "This is pure mass murder," one commentator said. "War of terror against America - live'' read the bottom-of-screen banner on ZDF television.

Broadcasters broke into programming all across Europe and in Japan and India.

"It's incredible. I thought I was watching a Hollywood movie," said Hong Kong school teacher Doris Tang.

"This is perhaps the most audacious terrorist attack that's ever taken place in the world," said Chris Yates, an aviation expert at Jane's Transport in London. "It takes a logistics operation from the terror group involved that is second to none. Only a very small handful of terror groups is on that list."

He named Osama Bin Laden and Hezbollah as possible suspects.

But Professor Paul Rogers, of Bradford University's Peace Department in central England, warned against assuming Middle East extremists were behind the tragedy.

"We've been here before," he said. With Oklahoma, everybody assumed it was Middle East, then it turned out to be home-grown Timothy McVeigh.

The World Trade Centre has been targeted before, and was the scene of a massive van bomb in February 1993.

"It suggests a well-organised paramilitary group is involved seeking to inflict great financial and psychological harm on the United States," Rogers said. "The World Trade Centre is hugely symbolic. It is a giant financial complex with great psychological and political prestige."