The government has assured its people and the Olympic community that heavy security will ensure a secure games. But its clampdown has smothered a broad array of groups, many with grievances against the government but without a history of violence.
Among the potential troublemakers Chinese security specialists have identified are Tibetan separatists, who staged occasionally violent protests last spring; members of the banned Falun Gong spiritual movement and unemployed workers.
Stirring the latest concerns were videotaped threats purporting to be from an Islamic militant group. They surfaced last week in the name of the Turkistan Islamic Party _ a group Chinese and Western terrorism experts say is an offshoot of a secessionist group from China's Central Asian frontier with ties to al-Qaida.
In it, hooded men stood in camouflage fatigues with Kalashnikovs and claimed responsibility for explosions in four cities in Western China in recent months, including two bus bombings last week in Kunming city that authorities said killed two people and injured 14.
One militant, identified by the Washington-based monitoring group IntelCenter as commander Seyfullah, warned athletes and spectators "particularly the Muslims" to stay away.
"Our aim is to target the most critical points related to the Olympics. We will try to attack Chinese central cities severely using the tactics that have never been employed," he said.
Chinese police immediately played down the threat, saying the explosions in Chinese cities claimed by the group were not the work of terrorists.
Still Beijing is being emptied of political critics, underground Christian organizers and ordinary Chinese who come to the capital to protest local government injustices.
Plainclothes security agents surprised rights campaigner Hou Wenzhuo at a cafe on May 30, putting a hood over her head and holding her in an undisclosed detention center for 17 days.
Among their chief concerns during interrogations, she said, were plans for a "human rights torch relay" organized by an exiled Tiananmen Square democracy movement figure and whether Chinese at home might get involved.
"The government is worried that this 'human rights torch' will detract attention from China" and the Olympics, Hou said. "They didn't beat me, but there are different kinds of intimidation."
Officials in charge of security have denied they are rounding up peaceful critics and have defended their actions as necessary, given global terrorism's scope and the publicity attacking the Olympics would bring.
To squelch any threat, Chinese leaders are mobilizing an army of security many times greater than previous Olympics _ 110,000 police, riot squads and special forces, augmented by more than 300,000 Olympic volunteers and neighborhood watch members.
"Through all kinds of efforts and by relying on the support and cooperation from the international society and the general public, we are confident we can deal with all the threats and risks and challenges," Liu Shaowu, director of security for the Beijing Olympics organizing committee, said last week.
President Hu Jintao told fellow communist leaders over the weekend that "the task of hosting a safe Olympic Games is as heavy as Mount Tai and everyone shares the responsibility."
The hyper-charged security, however, has put some Western governments on edge, caught between a desire to cooperate on terrorism threats and concern about aiding the policing of peaceful dissent. U.S. and other European governments complain they have offered information but that Chinese police give little in return.
"The Chinese definition of security threat is pretty broad, and in the context of the Olympics, it encompasses anyone who might eek to 'disrupt' the games," Drew Thompson, director of China studies at the Nixon Center in Washington, said in an e-mail.
For the communist regime, "it's not about terrorism. It's about security," said one Western diplomat, who asked not to be identified because he was not authorized to speak to the media. "It's the current reason for expanding the entire scope of the police state."
Chinese and Western terrorism experts agree the threat from terrorist groups, particularly of the militant Islamic stripe, is real. Hardly a month has passed this year without the government reporting it had disrupted a terrorist plot. But with so much effort focused on Beijing, terrorists may be seeking more vulnerable targets.
"The chances of attacks on Olympic areas are very unlikely," said Rohan Gunaratna of the International Center for Political Violence and Terrorism Research in Singapore. "But there could be attacks elsewhere in the run-up to the Olympics to spoil the mood of celebration."
Li Wei, director of the Center for Counterterrorism Studies, which has ties to China's spy agency, said his center has pinpointed five distinct threats: international terrorist groups like al-Qaida, the domestic version fighting to end Chinese rule in far western Xinjiang province, Tibetan separatists, the Falun Gong spiritual movement and ordinary people with grievances against the government or society.
While Li said the Tibetans and Falun Gong are not known for violence, radicals in their midst might lash out. Followers of Falun Gong, a meditation practice suppressed nearly a decade ago after drawing millions of followers, might turn to self-immolation, poisonings and other retaliatory acts if ordered by their leader, believed in hiding in the U.S., he said.
Groups fighting to end Chinese rule in Xinjiang, or what some Muslims call East Turkistan, are singled out as the most likely threat. A rebellion by the indigenous Muslim Turkic people, the Uighurs, has simmered for decades, with some fighting in neighboring Pakistan and Afghanistan.
One group, the East Turkistan Islamic Movement, was based in Afghanistan before the U.S. invasion and is listed as a terrorist organization by the United States. After its leader was killed in 2003, members reorganized into similar groups, including the Turkistan Islamic Party, and received training from al-Qaida in Pakistan's tribal area abutting Afghanistan, said Gunaratna and other terrorism experts.
Gunaratna estimates this hard core numbers around 100. Aside from its recent videotaped threat, the Turkistan Islamic Party released a statement in April calling for biological weapons attacks against China and has posted an Internet video guide on assembling a truck bomb, the IntelCenter said.
Commander Seyfullah's claim to have carried out recent explosions has raised doubts about the group's reach.
Police, cited by Xinhua, said the bus explosion in Shanghai in which three people died was caused by an oil fire and the Wenzhou explosion by a debt-ridden gambler, while there's no evidence to connect the Kunming bus bombings to terrorism.
"Although the Turkistan Islamic Party claimed that they were responsible, I personally think that it's all bluff and bluster," said Li, the counterterrorism expert.
Those explosions, Li and others said, were most likely caused by "lone wolves" _ disgruntled individuals and the hardest threat to guard against. Li pointed to the bombing in a park at the 1996 Atlanta Olympics that killed one and injured 111 and was found to be the work of an anti-government extremist.
That "type leaves no clues but only a hot head," said Li.
Associated Press reporter Anna Johnson in Cairo contributed to this report.