The incident happened in Rainham Road South just before 5.50am today (September 10).
The pedestrian, a man in his 30s, was taken to hospital where his injuries were deemed potentially life-changing.
Two other people were checked over by paramedics at the scene but did not need to go to hospital.
Enquiries into the crash are ongoing and police are appealing for anyone with information to contact 101 quoting CAD 913/10SEP.
London Fire Brigade said crews were returning to station from another incident when they were called to the crash and quickly arrived on scene.
Firefighters provided immediate care to the pedestrian before paramedics took over.
In total, two fire engines from Dagenham Fire Station attended alongside ambulance crews and a paramedic in a fast response car.
The A1112 Rainham Road South was closed in both directions between Crown Street and Sandown Avenue but has since reopened.
