A spectacular opening ceremony marked the launch of the Beijing Olympics as the Games began amid a massive security operation.

Worries about smog and protests over human rights were temporarily forgotten as the eyes of the world gazed in awe at the stunning extravaganza.

More than 90,000 spectators packed the Bird's Nest stadium to witness the start of the games.

US president George Bush, Russian prime minister Vladimir Putin and French president Nicolas Sarkozy were among the dignitaries present as Hu Jintao, president of the People's Republic of China, launched the epic three-and-a-half hour opening ceremony.

It began with a dramatic blast of fireworks as 14,000 performers put on a show which must have been daunting for observers from the London 2012 games organising committee.

With a potential TV audience of billions, it was the biggest and mostly costly opening ceremony in the history of the Games and is unlikely to ever be matched.

In his speech at the end of the ceremony, Jacques Rogge, president of the International Olympic Committee, said: "For a long time, China has dreamed of opening its doors and inviting the world's athletes to Beijing for the Olympic Games.

"Tonight that dream comes true.

"Beijing, you are a host to the present and a gateway to the future. Thank you!"

Mr Rogge also called on the athletes to embrace fair play.

He added: "Please compete in the spirit of Olympic values: excellence, friendship and respect. Dear athletes, remember that you are role models for the youth of the world. Reject doping and cheating.

"Make us proud of your achievements and your conduct."

A blast of fireworks began the ceremony, followed by 2,008 drummers playing a fou - a Chinese percussion instrument - in perfect unison.

The spectacular continued with 29 giant firework "footprints" - representing the number of modern Olympic Games - from the centre of Beijing to the stadium.

A fantastic light-show followed, with giant illuminated Olympic rings being raised up from the floor of the stadium and then a series of visually stunning acrobatic, artistic and aerobatic displays.

The climax was a three-minute display involving 30,000 fireworks that almost defied imagination.

The flamboyant ceremony was conducted amidst the tightest of security precautions as the Chinese authorities deployed an extra 100,000 police and soldiers on the streets.

Beijing's international airport was closed for the duration of the ceremony and a no-fly zone imposed above the city.

The imposing Bird's Nest stadium was protected by rows of tall fences, surveillance cameras and anti-aircraft missile batteries.

The build up to the Games has been dogged with concerns over air quality in Beijing and China's human rights record.

Some measures of pollution were almost four times the World Health Organisation recommended level and visibility in the Chinese capital today was as poor as it has been for a fortnight.

The traditional torch relay bringing the Olympic flame to the host nation has been marred with a series of protests across the world.

Two British Free Tibet campaigners returned to the UK yesterday after being arrested for unfurling flags and banners outside the Beijing stadium.

More than 40 Olympic athletes have signed an open letter to the Chinese president condemning the country's human rights record.

Backed by Amnesty International in Germany, the letter calls on Hu "to protect freedom of expression, freedom of religion and freedom of opinion in your country, including Tibet".

In London, a raucous protest was held outside the Chinese Embassy as the opening ceremony commenced.

Demonstrators from Tibet, Zimbabwe, Sudan and Burma joined forces to protest against human rights violations and China's hosting of the Olympics.

The crowd chanted "Stop the killing", "Shame on China" and "Free Tibet - Chinese out" and carried banners, one bearing the slogan "Beijing Olympics genocide Olympics".

One protester, Tibetan Liawang Tsang fled his homeland with his family nine years ago in order to escape from the Chinese regime.

He said: "We are here together today because we need the Chinese government to talk about human rights, the situation in Tibet and media freedom.

"The Olympics shouldn't have been offered to China on the basis of their human rights record, but from this there have been positives as the attention of the world is now on China and their human rights record is in the spotlight."

Olympics Minister Tessa Jowell, who is attending the Games, said she hoped the global scrutiny of China during the Beijing Games would lead to lasting improvements in human rights.

Many people have been caught up in the Olympic spirit with thousands of viewers watching the opening ceremony on giant screens set up across the UK.

Hundreds packed into central London's Trafalgar Square to watch the event.

South African Michelle Nymphe, who lives in Hendon, north London, said: "I've seen the whole opening ceremony. The atmosphere in Trafalgar Square has been very good and the ceremony has been spectacular, very well done.

"I thought Sydney was good, but this has been better. The lights display and the special effects were amazing."

The Games will run until August 24.