The report came as China ramps up security before the Olympics, which are set to begin on August 8 in the capital Beijing and several other cities including Shanghai, amid what China has described as an unprecedented terror threat.

''We have obtained information that international terrorist organisations would likely launch an attack against an Olympic venue in the city during the Games,'' Xinhua news agency quoted the head of Shanghai's Olympic security office, Cheng Jiulong, as saying.

''We have staged raids and cracked a group of terrorists,'' he said.

The Xinhua report did not specify when the operation had taken place, or how many suspects had been detained.

Mr Cheng said the Olympic football venue in Shanghai, as well as players' housing, were now ''at a safe level,'' adding, ''But the threat of terror attack still exists.'' Chinese authorities have warned of an unprecedented threat to the Olympics, particularly from its restive Muslim Xinjiang region in the north-west, and have said the security of athletes and tourists is their top priority.

Critics, however, say that China's communist rulers are exaggerating the security threat to clamp down on any form of protest during the Olympics. Shanghai is set to host 12 Olympic football matches during next month's Games.

The venue has been closed since Sunday, with police conducting around-the-clock patrols.

Police had been placed on a ''crisis'' footing as part of efforts to ensure public safety in the city, the report said. AFP