Police shut down the bustling International Bazaar in the capital of China's restive Muslim region of Xinjiang on Friday amid threats from an Islamic group that attackers might target buses, trains and planes during the Olympics.

A sign at the entrance of the bazaar in Urumqi did not explain why the area, surrounded by mosques with minarets, was off limits as the country prepared to kick off the Summer Games thousands of miles away in Beijing.

But one of the many security guards in the bazaar's plaza, which was marked off with crime scene tape, told an AP reporter, "The area is closed because of a possible terrorist attack. It's just a defensive measure."

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

The e-mail, written in Japanese and received in the early afternoon, urged the airlines to suspend its flights or the writer would "bomb the aircraft," said Transport Ministry official Fumio Yasukawa.