It would have created a substantial fireball and basically engulfed anyone that was near the van

Peter Fararr Chief inspector

The man with the device then did something to it, before he and the gunman got out of the van and ran off.

When the driver and the other man reached a police checkpoint at the Grosvenor Road roundabout, they raised the alarm.

A security source said that while they were keeping an open mind on who was responsible "an obvious starting point must be republican dissidents".

The dissident groups are opposed to the peace process in Northern Ireland have continued to make attacks on the security forces in the province.

Chief Inspector Peter Farrar said the perpetrators had no regard for human life.

Fire ball

"If that had gone into the city centre it would have created a substantial fireball around that van and basically engulfed anyone that was near the van causing horrendous injuries," he said.

"You have to consider that this was about 1200 BST, or just after, on a Friday lunchtime and the people who did this obviously do not care about any sort of injuries or anyone's welfare within the city centre."

Army technical experts were called to examine the device which they said would have caused a fire ball if it had exploded.

The police operation to intercept the van caused traffic disruption in the area which leads on to the M1 motorway.

Police at a vehicle checkpoint on the Grosvenor Road saw two men running away from the van.

Meanwhile, another security alert on Wellington Place in the city centre caused traffic disruption a short time later.

Shops and offices were evacuated, but the alert was declared a hoax.