Perkins Great Eastern run was cancelled just half an hour before the event was due to start following reports of a man acting suspiciously.
A statement from organisers said: "In the interests of safety for our participants we took the decision to cancel the half marathon half an hour after its original start time.
"This was because of police information that a man was acting suspiciously close to the route."
In the event it was a false alarm.
Among the hundreds of runners at Peterborough was Katie Mowatt of Chatteris who would have been taking part for the first time.
She said: "I wasn't doing it in aid of a charity this time due to it being my first ever half marathon.
"I am totally gutted to have not run but 100 per cent signing up for next year."
Assistant chief constable Dan Vajzovic was also due to have run: "I was ready to go and looking forward to the challenge.
"However, it was the right decision for the event organisers (working closely with police colleagues) to cancel the race. Public safety is paramount."
A police statement said: "We were called with reports of a man acting suspiciously in Alexandra Road, Peterborough this morning which ultimately resulted in the Great Eastern Run being cancelled.
"We take all calls extremely seriously, even more so in light of recent events across the country, and as a result a major firearms response was launched.
"Following a comprehensive investigation it is believed that the witness was acting in good faith but has misinterpreted what was seen.
"We would like to thank the public, the race goers and the organisers of the Perkins Great Eastern Run for their patience and support today.
WERE YOU AMONG THOSE READY TO RUN? Send us your photos please to john.elworthy@archant.co.uk
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