The first injuries happened when a car ploughed into a group of pedestrians in Essex, leaving one of them in a critical condition.
Police said the vehicle collided with six people in Furtherwick Road, Canvey Island, at around 4.10pm yesterday.
The East of England Ambulance Service said the casualty in a critical condition and another victim with a potentially life-changing leg injury had been flown to London hospitals.
A third person was taken to Basildon and Thurrock Hospital in a stable condition, and two others were taken to Southend Hospital in a serious condition.
Jon Moore, of the ambulance service, said: “The emergency services and critical care teams worked extremely quickly and professionally together at this incident to ensure patients received the right care as swiftly as possible.”
Anyone with information is asked to contact police.
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Separately, a 19-year-old man has been arrested after a car collided with pedestrians in Blackpool town centre just before midnight.
Three people were treated by the North West Ambulance Service. Their injuries are not thought to be life-threatening.
The incident is not being linked to terrorism, and police have appealed for anyone with footage to contact them on 101 quoting log 1862.
The Press Association contributed to this report.