A half-marathon in Peterborough was cancelled because of a false alarm which sparked a "major" armed police response.
Organisers initially cancelled the Perkins Great Eastern Run just 30 minutes after it was due to begin, as a man was seen acting suspiciously close to the route.
Police said a "major firearms response" was launched, but an investigation found a witness had "misinterpreted" what they had seen.
Cambridgeshire Police was called at 9.50am to Alexandra Road, close to the event's route through Peterborough.
The event, which was due to begin at 10.30am, was cancelled "in the interests of safety", organisers said, which police said was the right decision as "public safety was paramount."
“We take all calls extremely seriously, even more so in light of recent events across the country,” a Cambridgeshire police spokesperson said.
“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.”
Cambridgeshire police’s assistant chief constable Dan Vajzovic was among the thousands of runners ready to take part in the race.
He said: “I was one of the runners who had spent months preparing for today’s event.
“I know that 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.”
Organisers said runners will be contacted next week to “confirm the process for the full refund of entry fees”.
Those who were unable to take part can also defer their entry for the 2020 event, organisers added.
Thousands of people were expected to take part in the event, which was organised by Peterborough City Council.
Race director Jon Marsden said: "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."
In a statement, the organisers said: “In the interest of safety for our participants we took the decision to cancel the half marathon half an hour after its original start time because of police information that a man was acting suspiciously close to the route.
“We are disappointed, as we are sure you are.”
Last year’s Perkins Great Eastern Run saw 3,452 people finish the race.
Assistant chief constable Mr Vajzovic added: “Public safety is paramount.
“I will be back next year, looking to achieve a personal best and I hope all the other runners will be too.”