Thomas Worby, of Milton Keynes, Buckinghamshire, had joined a protest on Saturday at this season's final meeting of the Cambridgeshire Hunt, at Waresley. Relatives said his parents had warned him to be careful.
The 53-year-old driver of the vehicle involved, which was travelling along a farm track at the time of the incident, was questioned and released on police bail, pending further inquiries. Police said the lorry would undergo a forensic examination.
A post-mortem examination revealed that Thomas died of severe internal injuries.
Detectives said they were most concerned that children, 'some as young as 13', attended hunt protests. They aim to try to trace and interview everyone who was present at the time. The Hunt Saboteurs' Association said yesterday that the incident occurred after the hunt had stopped for the day.
In a statement, the Cambridgeshire Hunt said that it 'much regrets this fatality'.
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