"Secretary of State Colin Powell informed Greek Foreign Minister Petros Molyviatis that, due to the press of business in Washington, the Secretary would not be able to travel to Athens to attend the closing ceremonies of the Olympic Games," spokesman Richard Boucher said in a statement.

Powell's planned trip to Greece Saturday through Monday had sparked protests with hundreds of demonstrators marching in central Athens on Friday to denounce his arrival.

A huge banner bearing the message "Powell killer go home" was also draped over a side of the ancient Acropolis hillside overnight by militants of the Greek Communist Party.

"Don't forget that civilians are being slaughtered in Najaf and a wall is being built in Palestine," read a message on the banner that was written in English and Greek. It referred to the situation in Iraq and Israel.

Hours earlier, hundreds of demonstrators surrounded by a large number of police officers had rallied in central Athens carrying banners that read "Powell Out" and "US out of Iraq".

Riot police meanwhile used tear gas to disperse a group of anti-globalization militants trying to march to the US embassy in Athens. The protesters had planned to use a road earmarked only for official Olympic cars.

A peaceful rally also took place in Greece's second-biggest port city of Salonica.

During his brief trip, Powell was to have held talks with Prime Minister Costas Karamanlis.

"Secretary Powell called the foreign minister Friday, and expressed his congratulations to the government and people of Greece for hosting a spectacular, safe, and successful Olympics. He proposed visiting Greece in October," Boucher said.

The Greek foreign ministry said the cancellation was not related to the protests and specified that Powell will visit Greece in the first half of October.

But the organisers of Friday's rally insisted it was them who had thwarted the visit. "For what other reason could the visit have been cancelled?" said Petros Konstantinou of "Genoa 2001," an anti-globalization group that co-organised the rally.

"It's a huge success of the anti-war movement and a sign to US activists that we can win," Konstantinou told AFP, adding protestors would be out on the streets again in October to prevent Powell from coming.

"They shouldn't fool themselves. In October we can muster much more people".AFP