Thousands of runners were called away from a half marathon at the last minute as a major armed police operation was launched over a “suspicious” man.
About 3,500 people had been expected to take part in the Great Eastern Run in Peterborough earlier.
The race had been due to start at 10:30 BST and was delayed, then called off in the “interests of safety”.
Cambridgeshire Police launched a “major firearms response” but later said no crime had been committed.
The force declined to say what suspicious activity it had believed was happening.
It said a witness had acted in good faith but “misinterpreted” what had been seen, in Alexandra Road, close to the course, at about 09:50 BST.
Assistant Chief Constable Dan Vajzovic, who was due to take part in the half marathon, said: “I know the cancellation will have been incredibly frustrating for all those involved.
“However, it was the right decision for the event organisers (working closely with police colleagues) to cancel the race.”
He said public safety was paramount.
The force said the information was passed on to the race director who decided to cancel the event.
The organisers tweeted that they “took the decision to cancel the half marathon half an hour after its original start time” because of the police information.
Race director Jon Marsden said he and his fellow organisers had been left “shell-shocked” by the false alarm and had been worried about people waiting in the cold.
“We are really disappointed that we had to cancel the event and we do apologise to all the people that had booked their run with us today,” he said.
“We had to consider the issues around the runners who were waiting and starting to suffer from hypothermia, and also we had to consider the road closures because we had guaranteed to city residents that we would re-open by 2pm.”
He added that runners would be contacted in the next two to three days about refunds or deferring entry.
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