ITV News Anglia's Matthew Hudson was at the event
Cambridgeshire Police has revealed that the Great Eastern Run was cancelled over a false alarm.
The half marathon was cancelled after a man was seen acting suspiciously close to the route- on Alexandra Road- leading to a major firearms response.
Cambridgeshire Police say the witness was acting in good faith but 'may have misinterpreted what was seen' adding that it was the right decision to cancel as 'public safety was paramount.'
"The circumstances dictated that we had to make a very, very late call and decision prior to the scheduled start time and the consequence of that would have meant the race at best would have been delayed significantly and of course we've got roads that we've obviously got to open and we also had a lot of our runners who were waiting for the start of the race who were starting to get extremely cold."
Around 4,000 runners were taking part, in one of the biggest events in the city.
Organisers have apologised to everyone taking part or watching the event.