A 23-year-old man was taken to hospital as a priority with a head injury after cars were struck with concrete near South Woodford this morning.
Police were called to the A406 near to the Waterworks Corner roundabout, near Epping Forest, at 3.20am.
They were responding to reports that debris had been thrown from a railway bridge onto the road.
Paramedics also attended the scene and treated two drivers.
The 23-year-old was taken to the Royal London Hospital, Whitechapel.
There are no further details about the second driver at present, who was taken to Whipps Cross Hospital.
A total of five vehicles were damaged.
Police are investigating.
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