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 kilometers 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. "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. A flight carrying 70 people from Nagoya to Shanghai was forced to return to Japan after the threat was received, and it landed safely, he said. Two other jets underwent safety checks and departed Fukuoka, in southern Japan, one headed to Beijing and one to Shanghai, Yasukawa said. With China's communist government saying they have thwarted attempts to disrupt the Olympic Games, more than 100,000 soldiers and police are guarding Beijing and other Olympic co-host cities. The capital's iconic Tiananmen Square was closed off to visitors much of Friday as Chinese President Hu Jintao hosted world leaders at a luncheon banquet at the Great Hall of the People ahead of the opening ceremony. On Thursday, a videotape purportedly made by the Turkistan Islamic Party - a militant group seeking Xinjiang independence - was released with threats to launch attacks during the Olympics. "Choose your side," says the videotape's speaker, grasping a rifle and dressed in a black turban and camouflage with his face masked. "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," he warns Muslims, according to a translation by the SITE Intelligence Group, a US operation that monitors militant organizations. 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. 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. The new video claims the communist government's alleged mistreatment of Muslims justifies holy war. It accuses China of forcing Muslims into atheism by capturing and killing Islamic teachers and destroying Islamic schools, according to the SITE. It also says China's birth control program has forced abortions on Muslim women. 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. But 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." 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. Urumqi has been on high alert. Security guards were checking bags at the entrances of hotels, department stores and discos in the busy city, which unlike other Xinjiang cities is predominantly Han Chinese - the nation's largest ethnic group. In the past 50 years, Han Chinese have been flooding into the region, creating another sore point with the Uighurs. 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. Xu Zhongcheng, an expert on counterterrorism and organized crime at Shandong Police College in eastern China, said security experts were taking the threats seriously, though more as "psychological warfare than real acts." He added, "The more terrorist threats, the more closely the Chinese people will stick together."