Oded Balilty, Associated Press
Chinese paramilitary police officers provide security Friday at the National Olympic Stadium, also known as the Bird's Nest, during the opening ceremonies for the 2008 Summer Olympics in Beijing.

URUMQI, China — 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.

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-Qaida.

A sign at the entrance of the bazaar in Urumqi did not explain why the area 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.

In the run-up to the Olympics, China's human rights record came under intense scrutiny, with some activists urging a boycott over the issue. The Turkistan Islamic Party's new video alleged China mistreated Muslims and claimed this justified a holy war.

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.

Last month, the Turkistan Islamic Party issued videotaped threats and claimed responsibility for a series of bus bombings in China in recent months.