"We obtained information that international terrorist organizations were likely to launch an attack against an Olympics venue in the city during the Games," said Cheng Jiulong, security head for the Games in Shanghai, a co-host Olympics city along with Beijing, the Shanghai Daily and the state-run Xinhua news agency reported.

Chinese authorities have been tightening security for months in the run-up to the Olympics, focusing especially on the far west Xinjiang region near Pakistan, where Islamic separatists have been fighting for autonomy. Dozens of people from these regions have been detained, accused of being terrorists.

Cheng said police staged raids and nabbed the alleged terrorists in Shanghai but did not disclose how many were arrested, where they were found or details of their background, the report said.

He said the Olympics venue and sites are safe, but added, "However, the threat of terrorism remains as some international terrorist organizations have threatened to launch attacks against the city."