The Great Eastern Run half marathon was cancelled this morning after a man was seen acting suspiciously near the race route.
Organisers left hundreds of runners disappointed when they decided to call off the event half an hour after it was due to start in Peterborough.
The decision was taken by Cambridgeshire Police after they received a call at 9.50am of reports of a man acting suspiciously close to the eleven-mile milestone.
The Great Eastern Run half marathon in Peterborough was cancelled this morning after a man was seen acting suspiciously near the route (police car pictured at the scene)
Hundreds of runners (some pictured) were left disappointed after organisers called off the event after rumours of a bomb in Peterborough this morning
Police said a 'major firearms response' was launched but an investigation found that the witness had 'misinterpreted' what they had seen.
A spokesperson for Cambridgeshire Police said: '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.'
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'.
Cambridgeshire Police said they advised organisers to cancel the race over fears for the safety of runners (pictured going home) and spectators
Those who were unable to take part will also be able to defer their entry for the 2020 event.
Thousands of people were expecting to take part in the event which was organised to start at 10.30am by Peterborough City Council.
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.'
This runner was hoping to race money for children's charity the NSPCC
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.
There were mixed reaction of social media after the race had been cancelled.
Some users praised police for their actions as others branded it as 'embarrassing'.
Originally, organiser Dick Hughes told the BBC: 'Basically all I know is what I've been able to announce. It is sad.
'It is a police matter, and the police have advised us to cancel for the safety of the spectators and the runners.
'Last year we had an incident that meant we had to postpone the start, we've always been proud of the fact that since 2006 we've always started at 10am for the half marathon.
'Last year we couldn't we were 10 minutes late, and now we've got this one, but this one is far more serious.
'It's a very disappointing day for Peterborough.'
A police officer is pictured surveying the scene in Peterborough this morning
Cambridgeshire Police said in a tweet: 'We were called at 9:50am this morning with reports of a man acting suspiciously in Alexandra Road, Peterborough close to the route of the @PerkinsGER.
'This information was passed to the race director who made the decision to cancel the race #GER #greateastern #greateasternrun'.
The official Great Eastern Run Twitter account posted: 'Owing to a Police incident the Perkins Great Eastern Run has been cancelled.
'Further updates will be provided when we can. Apologies to all and thank you for your patience.'