Updated

Police shut down a bustling bazaar in the capital of China's restive Muslim region of Xinjiang Friday, tightening security there after an Islamic group seeking independence for the area threatened to attack buses, trains and planes during the Olympics.

In Tokyo, an anonymous bomb threat e-mailed to Air China's Tokyo offices forced a passenger jet to make an emergency return to Japan, the Japanese Transport Ministry said. Four other flights were delayed.

Xinjiang's regional capital Urumqi was on high alert with security guards checking bags at the entrances of hotels, department stores and discos in the busy city the day after a new Olympics threat from the Turkistan Islamic Party — a militant group seeking independence for Xinjiang. A videotape purportedly made by the group warned Muslims to avoid being on planes, trains and buses with Chinese during the Beijing games beginning Friday.

The Turkistan Islamic Party is believed to be based across the border in Pakistan, where security experts say core members have received training from Al Qaeda.

A sign at the entrance of the bazaar in Xinjiang's regional capital Urumqi did not explain why the area, surrounded by mosques with minarets, was off limits as the country prepared to kick off the Olympics thousands of miles away in Beijing. The area was marked off with crime scene tape.

"The area is closed because of a possible terrorist attack. It's just a defensive measure," one of the many security guards in the bazaar's plaza told The Associated Press. He spoke on condition of anonymity because he was not authorized to speak to journalists.

At a shopping plaza in Urumqi, one Uighur businessman who would only identify himself as Kurban, said he did not support violent groups and valued his relationship with Han Chinese, China's dominant ethnic group. But he said he also sympathizes with Uighurs who dislike the Chinese because so many Uighurs are forced to live in poverty and repression in the countryside.

"The countryside is tense and very poor," he said. "But if I talk about them anymore, the police will come and take me away."

At noon prayers at one of the city's largest mosques, the crowd spilled out onto sidewalks, where men knelt on prayer rugs. A street-cleaning truck blasting loud music roared by the crowd spraying water that soaked the backs of worshippers.

In Tokyo, an anonymous e-mailed bomb threat, written in Japanese and received in the early afternoon at Air China's offices, urged the airlines to suspend its flights or the writer would "bomb the aircraft," said Japanese Transport Ministry official Fumio Yasukawa.

"We suspect this is a threat related to the Olympics," he said, refusing to provide further details of the note or say whether any particular groups were suspected of sending it.

The threat forced a flight carrying 70 people from Nagoya, Japan to Shanghai, China to return to Japan where it landed safely. Two other jets underwent safety checks and departed Fukuoka, in southern Japan, one headed to Beijing and one to Shanghai, Yasukawa said.

China's communist government says it has thwarted attempts to disrupt the Olympics and more than 100,000 soldiers and police were guarding Beijing and other Olympic co-host cities. The capital's iconic Tiananmen Square was closed off to visitors much of Friday because Chinese President Hu Jintao was hosting world leaders at a banquet before the opening ceremony at the Great Hall of the People on the western side of the public square.

On the eve of the opening ceremony, U.S. groups that monitor militant groups reported the new video threat against Olympics from the Turkistan Islamic party.

"Choose your side," a man who says he represents the group says in the six-minute video, addressing 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," says the speaker, grasping a rifle and dressed in a black turban and camouflage with his face masked.

He said Chinese mistreatment of Muslims justified a holy war.

The man in the video spoke in the Turkic language of the Uighurs, a largely Muslim minority in the Xinjiang region and a translation by the SITE Intelligence Group, a U.S. operation that monitors militant organizations.

The sprawling, far-flung western region of Xinjiang has long been a source of trouble for China's communist government. The rugged, mineral-rich territory is populated by the Uighurs, a Turkic Muslim minority that has had tense relations with the Chinese. Many Uighurs favor independence or greater autonomy for Xinjiang, which takes up one-sixth of China's land mass and borders eight Central Asian countries.

Last month, the militant group issued videotaped threats and claimed responsibility for a series of bus bombings in China in recent months. The latest video features graphics similar to ones used earlier: a burning Olympics logo and an explosion imposed over an apparent Olympic venue.

On Monday, assailants killed 16 border police and wounded 16 others in the Xinjiang city of Kashgar when they rammed a stolen truck into the group before tossing homemade bombs and stabbing them. Chinese authorities called the raid a terrorist attack and said they had arrested two men who are Uighurs. Authorities have called the men terrorists, but officials have released no evidence linking them to a specific group.