A worldwide television audience saw a grand opening ceremony lift the curtain on the biggest sports show on earth, as patrolling helicopters and troops underlined the Games are guarded by the largest security operation in peacetime Europe.

International Olympic Committee President (IOC) Jacques Rogge and Greek leaders have said everything humanly possible has been done to ensure the safety and success of the Olympics -- the first summer Games since the September 11, 2001 attacks.

Tens of thousands packed the futuristic new Olympic Stadium, centerpiece of a two-week festival of sport that was under threat from chaotic construction delays until the last minute.

After three hours of pageantry, former windsurfing gold medallist Nikos Kaklamanakis lit the Olympic flame.

He was by all accounts a late substitute for sprinter Costis Kenteris, the Olympic 200 metres champion who had plunged home fans into despair by missing an eve-of-Games drugs test and then crashing his motorbike, leaving his participation in doubt.

Rogge had earlier urged the athletes to set an example for world peace -- and forswear drugs -- before Greek President Costis Stephanopoulos declared the 28th Olympiad open.

"The country that gave birth to athleticism welcomes you," declared the announcer as the parade of athletes ended.

Russian and American astronauts sent video greetings from their space station far above the earth.

A heartbeat of drums had counted down to the opening moment and, amid torchlight and fireworks, a boy aboard a paper boat drifted across the flooded arena waving a Greek flag.

GODS, LASERS

Combining symbolism of the ancient Greece that created the Olympic Games nearly 3,000 years ago with 21st technology, giant sculptures appeared above the waters. A living frieze of Greek history paraded by, the voice of opera diva Maria Callas echoed across the stadium and lasers created a DNA helix.

Led by the Greek flag, athletes from the 202 competing nations then streamed in, with tiny Saint Lucia -- Agia Loukia in Greek -- taking alphabetical pride of place at their head.

Big cheers greeted the Afghan and Iraqi teams, a mark of sympathy for their nations' troubles. Contrary to some fears in the U.S. camp, the big American contingent met warm applause, as did the North and South Koreans who held hands, and Greece's old enemies the Turks who have lately been patching up relations.

Former U.S. President George Bush was in Greece to join world leaders in a show of unity behind the Olympic ideal of peace. Turkish Prime Minister Tayyip Erdogan said he would bring a message of global cooperation.

Among political and royal leaders present from around Europe and further afield were British Prime Minister Tony Blair, Spain's Queen Sofia and the presidents of Germany and Italy.

Organisers expected four billion to watch on television.

DOPING DRAMA

Hours before the ceremony, the IOC spared host nation major embarrassment by granting "Greece Lightning" Kenteris and Katerina Thanou more time to explain why they missed dope tests.

But a 72-hour extension for Kenteris and training partner Thanou, the 100 metres silver medallist, after they failed to show up for a disciplinary hearing on Friday was perhaps only a postponement of what could still turn into a national scandal.

The twists and turns of a drama in which the two were injured in the motorbike crash and ended up in hospital stunned Greece as television and radio stations broke into programmes with the latest news on the nation's heroes.

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Officials said neither was seriously hurt but had been ordered to stay in medical care for at least two more days.

"In order to ensure a fair process and give due consideration to the athletes, the disciplinary commission has decided to postpone the hearing until Monday," the IOC said.

The Greek National Olympic Committee said it would hold a special meeting on Saturday to discuss the missed doping tests and "all relevant developments".

After waiting in vain for the pair to leave hospital for the hearing, IOC drugs panel member Sergei Bubka, the Ukrainian former Olympic pole vault champion, said: "It was a doctor's decision. It wasn't really our decision."

Deepening the murk of a case that may leave doubt hanging over the pair like the sword of Damocles, Bubka said: "It was there on a medical certificate. It said these athletes should not be transferred anywhere for 48 hours."

Suspicion over Kenteris and Thanou delivered a severe shock to a country savouring one of its proudest moments.

Rogge said the fact the two were Greek national heroes would have no bearing on their fate.

The three-member IOC disciplinary commission has the power to rule a missed test is a failed test, carrying a two-year ban.

All of Greece was staggered by the news. Call-in show reaction swung from sympathy to stoicism to paranoia.

The daily Ethnos newspaper urged the sprinters: "Tell us the truth -- You owe it to all Greeks to prove you are clean".