There is a "real possibility" that the Beijing Olympics will be attacked by terrorists, the head of global police body Interpol has warned.
Speaking in Beijing, Ronald Noble said the Games could give "easy cover" to groups such as al-Qaeda.
He also warned that protesters who have disrupted the Olympic torch relay might also target this summer's Games.
China claims to have foiled several plots to attack the Olympics by Muslim separatists from Xinjiang province.
Mr Noble told a security conference in the Chinese capital: "Based on reports of thwarted plots in the Chinese media, including an attempt to bring down an airliner headed to Beijing, it seems clear that the threat has increased."
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He said the unrest in Tibet, and the protests that have followed, had given rise to "additional complications" for the organisers of the Games.
"When thwarted attacks are coupled with the recent violent protests viewed by us all worldwide, prudence requires us to recognise the real possibility that groups and individuals could carry on their protests at the actual Games," he said.
"These activities could range from disruptive behaviour, like blocking major transportation routes or infrastructure or interfering with competitions, to more violent acts like assaulting Olympic officials or athletes or destroying property."
He also said security services must be prepared "for the possibility that al-Qaeda or some other terrorist group will attempt to launch a deadly terrorist attack at these Olympics".
He said the influx of foreigners and the world's media could provide "easy cover" for terrorists and ensure any attack would get global coverage.
Mr Noble added that an Interpol team would be training Chinese officers in crisis management and major event operations before the Games.
