China has declared it is ready to stage one of the greatest Olympics ever, but a new terrorist threat, pollution concerns and human rights controversies hangs over final preparations.

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IN DEPTH: The Beijing Olympics

 As the world's best athletes poured into Beijing and the Olympic flame passed over the Great Wall, Games organisers sought to shift global attention to the opening ceremony and what they promised would be a sports spectacular.

"We have prepared for the Beijing Olympics for seven years and now we are ready ... we are very confident indeed that we will stage a successful Olympics," organising committee spokesman Sun Weide told AFP.

"Of course we hope that these will be a great Games, even the greatest."

For China, the Games are an opportunity to show the world how far it has come since the communists came to power in 1949 following a brutal civil war, and particularly the past three decades of phenomenal development.

The event offers to highlight China's social and economic transformation, similar to the 1964 Games for Japan and the 1988 Olympics for South Korea.

Swirling controversy

But the vast array of controversies that have swirled around the Olympics this year continue to bedevil the Chinese leadership, from the smog that has stubbornly hung over Beijing to human rights storms.

China has also warned that "terrorists" from home and abroad pose a massive threat to the event. This swung into focus late Thursday when a Muslim separatist group in China made a new video threat, warning Muslims to keep their children away from the Games.

The Washington-based IntelCentre, which monitors extremists, said the nearly six-minute video shows flames consuming a Beijing Olympics logo and an explosion over a venue for the competition. It featured a masked, turbaned speaker clutching what appears to be an assault rifle.

The SITE Intelligence Group, meanwhile, said the speaker identified himself as a member of the Turkistan Islamic Party (TIP), and warned Muslims: "Do not stay on the same bus, on the same train, on the same plane, in the same buildings, or any place the Chinese are."

Separatist threat

The TIP is an ethnic Uighur and Muslim organisation seeking to create an independent state out of China's heavily Muslim Xinjiang province.

In July, the IntelCentre said the group had taken credit in another video statement for a deadly bus bombing in Shanghai in May and warned of new attacks in China during the Olympics.

More than 100,000 security personnel are patrolling Beijing, anti-missile barriers have been set up near the "Bird's Nest" Olympic stadium, and the military and police are on guard around the country.

Critics have accused Chinese authorities of exaggerating or fabricating threats so that they have an excuse to silence critics, but Beijing has made no apologies for its crackdown.

'No Fun Games'

The suffocating security precautions China has employed have dampened some excitement, with a few critics dubbing the event the "No Fun Games".

A mixture of pollution and fog again cut visibility across Beijing on Thursday despite much publicised emergency measures to improve air quality.

The pollution has been particularly embarrassing for China because it has highlighted to the world one of the worst side-effects of its historic modernisation drive - massive environmental degradation.

China's human rights record remained under a fierce Olympic spotlight with over 40 athletes due to compete in Beijing sending a letter to Chinese President Hu Jintao expressing their concern over the issue.

Bush comments on human rights

US President George W Bush arrived in Beijing after delivering a speech in which he raised "deep concerns" about China's respect for human rights.

"The United States believes the people of China deserve the fundamental liberty that is the natural right of all human beings," Bush said in Thailand, triggering an indignant response from Beijing.

"We firmly oppose any words and deeds that use human rights and religion to interfere in other countries' internal affairs," foreign ministry spokesman Qin Gang said.

From Canada to Nepal and India to Germany, protesters decried China's human rights record. Candle vigils were planned in several Swiss cities, while the Free Tibet campaign was to protest on Thursday (AEST Friday) in front of the Chinese embassy in London.

More than 100 heads of state and other senior national leaders are expected to attend the Games, ensuring a current of political tensions will flow through the event despite China's leaders repeated efforts to dam the issue.

However, that is all expected to be at least briefly swept aside during the opening ceremony that will kick off on Friday at 8pm (2200 AEST).