"An efficient Olympic security command system is in place," Sun Weide, a spokesman for the organizing committee, told The Associated Press. "We're confident of holding a peaceful and safe Olympic Games."
Few details have been released about the alleged plot and an apparently unrelated attempt to crash a passenger jet last Friday.
The response underscores China's secretive, often repressive approach to a long-simmering separatist movement in its far West: Officials say grave threats require harsh tactics, and that they have the situation entirely in hand.
The government's failure to provide details and evidence, meanwhile, has fueled speculation it was overstating the security threat to justify tough measures at the Games - an event it hopes will boost its legitimacy at home and project a modern, progressive image abroad.
"When China has made allegations of terrorist activity, it doesn't back it up with evidence and restrictions make it impossible for independent investigators to verify," said Mark Allison, a Hong Kong-based researcher for Amnesty International.
As host, China has legitimate and serious safety concerns, Allison said. But Beijing's claims draw suspicion because of the communist regime's record of repressive policies and its regard for even peaceful protests as a threat to national security, Allison said.
"Without evidence, their claims are open to question," Allison said.
Wang Lequan, the top Communist Party official in the western region of Xinjiang, said Sunday that materials seized in a January raid in the regional capital, Urumqi, had described plans "specifically to sabotage the staging of the Beijing Olympics." Reports said two people were killed and 15 captured in the raid, along with weapons and extremist religious literature.
Wang said the plotters had been trained by and were following the orders of a separatist group based in Pakistan and Afghanistan called the East Turkestan Islamic Movement, or ETIM.
The United Nations and the United States have labeled the group a terrorist organization, although little is known about its size and capabilities. East Turkestan is another name for Xinjiang.
The ETIM is considered the most radical among the scattered groups fighting for independence for Xinjiang's 8 million-strong Turkic Muslim Uighur ethnic group. Riots, bombings and assassinations in the 1990s drew an overwhelmingly harsh Chinese response and few violent incidents have been reported in recent years, although Chinese forces who raided an alleged ETIM camp last year said they found homemade weapons and explosives.
Most experts say the actual threat to the Beijing Games from terrorism is low, although the event has become a magnet for critics of the government, ranging from free-speech advocates to activists over Sudan's troubled Darfur region.
Most security measures won't be known until closer to the Games, but they are said to include drafting in large numbers of additional security personnel from outside Beijing.
The city was installing thousands of surveillance cameras in and around Olympic venues and collecting intelligence on potential troublemakers.
Sun, the Olympic organizing committee spokesman, did not say whether additional security measures were being considered following revelations about the plot. He said a comprehensive security plan has been augmented by workshops, field training and full security rehearsals.
Also Monday, China's head of civil aviation confirmed Friday's incident aboard a China Southern Airlines flight from Urumqi to Beijing, but offered no new details.
The plane was forced it to make an emergency landing, but no damage or injuries were reported. The head of Xinjiang's regional government earlier described the incident as an attempt to crash the plane.