Armed police activated three rings of checkpoints around Beijing yesterday as part of the government's drive to ensure a safe Olympics next month. As the capital makes final preparations for the event, which is being held between August 8 and 24, police and paramilitary officers were stationed at checkpoints set up on the roads into the capital. They checked every vehicle coming in, leading to slower traffic but no severe jams, witnesses said. At checkpoints on the outer ring, officers inspected each vehicle with electronic equipment and sniffer dogs, witnesses said, delaying each by an average of 10 minutes. The outer ring seals Beijing off from the neighbouring province of Hebei on one side, and from the city of Tianjin on the other. The middle ring has been thrown around six suburban districts. The innermost security ring encompasses the city's eight districts. All but four of the 31 venues for medal competitions are within the innermost security ring. The municipal public security bureau said it would close all the checkpoints at the three security rings circling the city in an emergency. 'The checkpoints are set up to prevent dangerous materials and people from entering the national capital,' Xinhua quoted a senior security official as saying. Security officers also said bus passengers travelling to Beijing would have their identity cards and belongings checked from Sunday. If just one passenger failed to show a valid ID card, the bus and all its passengers would be refused entry to the city, a statement on the Ministry of Public Security's website said. Amid complaints from travellers about the repeated checks, Communist Party Politburo Standing Committee member Zhou Yongkang said police needed to be polite and efficient in their duties. 'During the Olympics security work, we should not only stick to strictness and details, but also ensure civilised and convenient inspections for people,' Xinhua quoted Mr Zhou as telling police at a checkpoint. With the central government saying terrorism is the biggest threat to the Games, special police armed with machine guns were patrolling Beijing's international airport yesterday. An airport police official said passengers would go through two security checks, the Beijing Morning Post reported. Security staff have also started duty at 90 sports venues, 110 designated hotels, the Olympic athletes' village and about 2,000 sites reserved for various Games activities.