Armed robbers on a motorbike ambushed a security officer on Thursday, stealing a secure cash box which he was carrying to a shop in Hendon.

The officer, who works for security firm Securicor, was making a delivery to the NatWest cashpoint inside Budgens in Brent Street, at the junction with Heriot Road, when the robbers drove down the pavement towards him just after 3pm, in what police believe was a planned attack. Having told him to drop the box, they threatened the guard with a handgun before he managed to take cover behind a letter box.

The Securicor officer then dropped the cash box, and his assailant picked it up, leapt onto the back of the motorbike and the two robbers sped away. His accomplice was not thought to be carrying a weapon.

Efrat Yosef, who works at the nearby Good For You childrens' toy shop, saw the robbery through the window. She said: "I saw two men on a motorcycle and this man with a black suitcase and I heard some screaming. Then one of the men tried to take his case and they struggled a bit and then he took out a gun, a small one, and the one who was carrying it dropped the suitcase, and the robber picked it up, got on the motorcycle and left."

Another eyewitness, who did not want to be named, said: "I think they were waiting for the Securicor guys because they were seen riding up and down, up and down. And they got away with it on their bikes."

Although the police will not confirm the amount in the suitcase, it may have been as much as £20,000, according to a Budgens employee.

A police spokesman said: "We believe this was a targeted attack, rather than an opportunist one."

The two men were dressed in dark clothing and were wearing crash helmets. After the robbery, they made off along Brent Street towards the North Circular Road. The motorbike was blue, and the registration number was Y37 NLE. Any witnesses are asked to contact the flying squad on 020 8358 1751, or Crimestoppers anonymously on 0800 555 111.

A Securicor spokesman said the company was offering a £5,000 reward for any information leading to the arrest and conviction of those responsible for the attack.