The tight security at the Beijing Olympics - the largest seen at a sporting event - was stepped up today after Islamist terrorists released a video warning of attacks on the Games.

The video, which appeared on a BBC Chinese website, showed a masked terrorist with assault rifle in front of a burning Olympic symbol urging Muslims: "Do not stay on the same bus, on the same train, on the same plane, in the same buildings, or any place as the Chinese."

The authorities shut down Beijing international airport and closed off the roads surrounding the Bird's Nest stadiumwhere the opening ceremony is taking place.

Uniformed officers and sniffer dogs surrounded the arena for final safety checks and the Army set up missile launchers in secret locations to fend off possible attacks from the air. About 300,000 surveillance cameras have been installed in the city.

The terrorists, who are calling for an independent Muslim state in the west of China, which they refer to as East Turkestan, are believed to be the same group which killed 16 Chinese policemen in Xinjiang province - which incorporates East Turkestan - this week.

More than 100,000 police officers and soldiers, backed up by spotter planes and helicopters, were on standby as US President George Bush and 80 other heads of state joined 91,000 spectators at the opening ceremony today.

The video's opening graphics show an explosion superimposed over one of the athletic sites and the terrorists identifies himself as Abdullah Mansour.

Speaking in the Uighur language, spoken by ethnic Turkic Muslims, he says: "We, members of the Turkestan Islamic Party, have declared war against China.

"We oppose China's occupation of our homeland of East Turkestan, which is a part of the Islamic world."

The violent opening graphics are the same as those in a video the group released last month, in which a masked man identified as Commander Seyfullah claimed responsibility for bus bombings in Kunming and Shanghai which killed five people and wounded at least 26.

In that video, he threatened violence against the Olympics and on 27 June he released a five-page statement calling for attacks on the Olympics and other targets in China, citing grievances against the government.

The statement said suicide bombers around the world were ready to carry out missions. "Particularly, go to the central Chinese cities and kill the top leaders, soldiers, police, prison wardens and accomplices," it said.

There are two Turkestan terrorist groups, the Turkestan Islamic Party and the East Turkestan Islamic Movement, or ETIM.

Chinese security services believe the ETIM were behind a series of attacks in Xinjiang. The latest was on Monday when two terrorists used a truck, homemade explosives and knives to kill at least 16 paramilitary police officers and wound 16 others in Kashgar, a desert oasis town.

Security experts blame the Chinese government for not keeping track of the rapid growth among splinter Islamic terrorist groups in western China.

In the weeks leading up to today's opening, the Chinese government has introduced tighter checks on residents and tourists. Visitors, even those who stay only one night, are expected to register at the local police station.

A global television audience of more than three billion people were expected to tune in to see the ceremony.

The Chinese have promised to stage "a show to remember", with more than 15,000 performers and 35,000 fireworks exploding over their heads.

A star Chinese athlete was being lined up to light the Olympic flame suspended 350 feet above the track.

But the spectacular ceremony could not end the continuing controversy over human rights. More than 40 athletes taking part urged the communist regime to respect human rights and freedom of religion, in an open letter addressed to the government just hours before the ceremony.

It marks fresh embarrassment for the host nation, which also faced criticism from Mr Bush and renewed protests in Tiananmen Square.

Signatories to the letter include men's 110m hurdles world record holder, Dayron Robles of Cuba, and US 400m runner DeeDee Trotter.

The letter calls on China's president, Hu Jintao, "to protect freedom of expression, freedom of religion and freedom of opinion in your country, including Tibet".

This year the British Olympic Association attempted to gag British athletes from commenting on politically sensitive issues by inserting a clause in their contracts telling them they faced being sent home if they offended the hosts.

But after protests, the association's chief executive, Simon Clegg, claimed there had been no intention to "restrict athletes' freedom of speech".