It is the country's single worst attack this year and marks a change of tactics to try to stop Iraqis joining the police and army, just as they are most needed. American combat troops will pull out of the country by the end of the month leaving a small force of 50,000 to train Iraq's security forces. However, there has been a spike in violence and al-Qaeda has warned of an escalation of attacks as the number of foreign troops falls. Yesterday's attack, in which a bomber sat among a queue of 1,000 Iraqi army applicants before detonating his explosives, was immediately blamed on al-Qaeda. The potential recruits were lining up outside an army base established at the Saddam Hussein-era defence ministry building in the centre of Baghdad. They were being allowed in 250 at a time, making the recruits and the monitoring soldiers and officers an easy target. There were about 1,000 applicants in all, a mark of the desperation for work of many young Iraqi men. It was the last day of the current recruitment round at the base, headquarters of the 11th Division of the Iraqi army. The recruits had been divided up into groups according to their level of education. Witnesses said the attacker had been sitting in the queue set aside for high school graduates for several hours. When an officer approached asking for identity papers, he triggered the explosive device, which was packed with nails. Bodies of the victims were left splayed and in some cases in pieces across the open space outside the base, some still clutching their applications, witnesses said. "After the explosion, everyone ran away, and the soldiers fired into the air," said Ahmed Kadhim, 19, a recruit who was unharmed. "I saw dozens of people lying on the ground, some of them were on fire. Others were running with blood pouring out."