Shanghai police are on crisis alert after they had 'struck down' terrorists, a top city official said yesterday. 'We have staged raids and cracked a group of terrorists,' Shanghai Public Security deputy director Cheng Jiulong told state media. He said police had 'obtained information that international terrorist organisations would likely launch an attack against an Olympic venue in the city during the Games.' Mr Cheng's comment is the first by a mainland policeman that China has cracked international terrorists targeting the Olympic Games. Officials have previously cited threats by Islamic terrorist groups but provided no evidence. Police also said terrorist attacks were still possible and announced a range of security measures to ensure the safety of Olympic soccer matches in the city, including cash rewards for tips. 'As we analyse the current situation, the threat of terrorist attacks is still present. International terrorist organisations have already issued threats to attack Shanghai.' Officials have previously cited threats by Islamic terrorist groups but provided no evidence. Shanghai government spokesman Chen Qiwei said yesterday security measures for the 12 soccer matches as part of the Games next month would be extremely strict. 'The police have already taken steps for a portion of those people who create a threat,' he said. Those considered a threat apparently include dissidents. Police detained lawyer Zheng Enchong , the city's best-known dissident, for six hours on Wednesday, marking the second time they had done so in four days. As part of the Olympic security drive, police had announced cash rewards of 10,000 yuan (HK$11,437) to 500,000 yuan for tip-offs, including information about 'illegal organisations' and the banned spiritual movement Falun Gong, the public security bureau said. Shanghai Stadium, the venue for the soccer matches, has been locked down since Sunday with 24-hour patrols. During the matches, the city will mobilise personnel and equipment to deal with potential bombings or even nuclear, chemical or biological threats. Shanghai has halted sales of explosives as well as toxic and radioactive materials until August 25, the day after the Olympics end. Security cameras will be installed on about 1,500 buses which pass near the stadium in western Shanghai.