Xinhua said four of the injured officers were in critical condition and two might not survive.

No organisation has claimed responsibility, but Xinhua quoted police sources as saying the raid was believed to be a terrorist attack. The two attackers, who have been arrested, were identified as Uygurs aged 28 and 33.

It was one of the deadliest apparent terrorist acts on the mainland in recent years and triggered further fears over the safety of Friday's gala opening of the Games despite a nationwide effort to boost security.

Officials said a curfew was imposed immediately after the morning attack but lifted later in the day.

Security checks at the airport in Urumqi also appeared to be tighter than usual.

Mainland stock markets ended down, following the track of regional markets. But analysts said the attack made recent weak sentiment even worse as Xinjiang-based firms were among the worst performers.

The public security department in Xinjiang confirmed the attack took place at about 8am in front of the Yiquan Hotel, which is about 100 metres from the border police division barracks.

One of the attackers drove a truck into a squad of more than 70 policemen who were out jogging, Xinhua said. The driver abandoned the truck after it veered and hit a utility pole, then threw an explosive at the officers. The second attacker, meanwhile, threw an explosive at the gates of the police station.

Fourteen policemen were killed on the spot and two died on the way to hospital.

Xinhua said police found 10 home-made explosives, a home-made gun and four knives in the truck. The driver of the truck blew off an arm when igniting an explosive and had undergone surgery at hospital, it said.

In the first official reaction to the attack, a spokesman for Bocog, the Beijing Olympics organisers, said they were confident and capable of holding a worry-free Games with the support of the Chinese government and assistance of the international community.

Sun Weide also said authorities were investigating any link between the attack and the Olympics.

International Olympic Committee president Jacques Rogge offered his condolences. 'If this attack is confirmed, the IOC offers its heartfelt condolences to the families and relatives of the casualties and the victims,' he told the South China Morning Post.

Dr Rogge sought to allay security fears among the international delegation arriving in Beijing. 'We are confident the Chinese authorities have done everything that is humanly possible to provide good security.'

Xinhua quoted the region's public security department saying it had received intelligence that the Uygur group known as the East Turkestan Islamic Movement (ETIM) was planning terrorist attacks in the first eight days of August.

Government officials have said ETIM poses the most serious threat to the security of the Games. The United Nations and the US consider the group a terrorist organisation.

Mainland anti-terrorist experts pointed fingers at ETIM for the attack. 'It is apparently a terrorist attack, which very likely to be linked to ETIM,' said Li Wei , director of the China Institute of Contemporary International Relations' Centre for Anti-terrorism Research.

Dilxadi Rexiti, spokesman for the exiled East Turkistan Information Centre, denied the accusation.

'It is groundless to accuse the ETIM of involvement,' he said, but added that Uygurs who had lost out under Beijing's rule were involved in 'armed struggle'.

Additional reporting by Minnie Chan and Shi Jiangtao