Thousands of runners were left on the starting line after a half marathon in Peterborough was cancelled following reports of a man “acting suspiciously”.
Police said a “major firearms response” was launched, but an investigation found that the witness had “misinterpreted” what they had seen.
Organisers confirmed that the Perkins Great Eastern Run, which would have seen some 4,000 competitors take part on Sunday, was cancelled following a “police incident”.
Runners were notified just 30 minutes ahead of the race’s start that the event had been called off “in the interests of safety”.
Cambridgeshire Police said its officers were called at 9.50am to reports of a man acting suspiciously on Alexandra Road, close to the event’s route in Peterborough.
Police said information was passed to the race director who then decided to cancel the event.
“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 and was due to start at 10.30am.
Race director Jon Marsden said: “We have had a lot of feedback from runners, they have been extremely understanding, they understood the predicament we found ourselves in.
“There are of course a number of runners who are very disappointed and I fully understand that. But the health, safety and welfare of participants in any event is paramount for us.”
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.”