Hampshire Police said a man in his 20s was arrested at the scene of the crash on suspicion of causing death by driving without due care and attention.
The double-crewed ambulance belonged to South Central Ambulance Service NHS Foundation Trust (SCAS) and was en route to Southampton General Hospital with the elderly patient.
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The prime minister was shouted at as she left the scene of the tragedy in which at least 79 people died.
Father-of-six Makram Ali, 51, was already receiving medical treatment as the attack unfolded. He died from multiple injuries, police say.
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