A courier was abducted at knifepoint by a gang of robbers who stole about 100 parcels.
The 26-year-old City Link driver was flashed by another car and made to pull over as he drove along Warren Road, Woodingdean, yesterday morning at about 8.30am.
When he stopped, a group of two to four men climbed aboard the green and yellow Mercedes Sprinter van.
One of them pulled out a knife and ordered the courier, from Shoreham, to drive to Underhill Lane, Ditchling.
The thieves then emptied the parcels into a getaway car and drove away, leaving the van and victim.
Police were called when the victim flagged down a passing motorist in the moments after the robbery.
He had been travelling from the City Link depot in Hailsham to make deliveries in Brighton. A member of the public called police at 9.34am saying he had been flagged down by the driver who said he had been robbed.
Officers have been conducting house-to-house enquiries in the search of witnesses and scouring CCTV footage covering the area.
A police spokeswoman said: "This was a traumatic experience for the driver. He has been left extremely shocked and distraught, although fortunately uninjured."
Detective Sergeant Kate Witt, from Brighton and Hove CID, said: "At this early stage, we have been unable to speak to the van driver so we have very limited information to go on. We are checking CCTV and I am confident that someone must have seen something.
"Warren Way at that time in the morning would have been busy with people on their way to work, on the school run or on their way to the nearby Tesco.
"The City Link van is a very distinctive yellow and green colour. Did you see the vehicle being made to stop or pull over? Did you see anyone acting suspiciously in the area, either on foot or in a vehicle?"
Officers are not sure if the getaway car had followed the van or arranged to meet there.
A spokesman for City Link said: "We can confirm that one of our drivers and his vehicle were involved in a serious incident this morning, which the police are currently investigating. Until the investigation is complete we are unable to comment any further."
Anyone with information is asked to call Brighton and Hove police on 0845 60 70 999 or Crimestoppers on 0800 555 111.
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