Organisers have cancelled a half-marathon in Peterborough after reports of a man acting suspiciously near the course sparked an armed police response.

The Perkins Great Eastern Run was called off 30 minutes after it was due to begin on Sunday morning “in the interests of safety”, organisers said.

The decision was taken after Cambridgeshire police was called at 9.50am to reports of a man acting suspiciously close to the event’s route.

Police said a major firearms response was launched, but an investigation found that the witness had misinterpreted what they had seen.

“We take all calls extremely seriously, even more so in light of recent events across the country,” a 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.”

The force’s assistant chief constable, Dan Vajzovic, was among the thousands of runners ready to take part in the race.

He said: “I … 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 would be contacted next week about the refunding 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 scheduled to start at 10.30am.

Jon Marsden, the race director, 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 3,452 people finished the race.

Vajzovic said: “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.”