A group of campaigners have expressed disbelief after Croydon South MP Richard Ottaway called the police about them delivering a petition to his office.
The Conservative, who was awarded a knighthood this month for his services to politics, asked for protection from officers ahead of 12 of his constituents arriving at his Brighton Road office on Friday.
The group, who were delivering a petition opposing a controversial lobbying bill, said they felt "intimidated" when confronted by officers as they arrived at around 5.30pm.
Liz Marsden, one of the campaigners, from had made an appointment to see Mr Ottaway.
But when the MP learned on Friday morning that other campaigners would be joining her, he called the police to express concerns about his safety.
Officers spoke briefly to the campaigners, including an 81-year-old woman, before leaving.
Ms Marsden, of Old Lodge Lane, said: "I spoke to Richard Ottaway that morning to say that there were going to be more people coming to show support for this petition and he was rather worried because he didn't know who they were.
"I said, well they're your constituents and clearly not threatening.
"He should be pleased that he has got constituents who are enthusiastic and interested who are willing to come out late on a Friday afternoon and take part in the democratic process. But he was not really interested in listening to his constituents."
The 39-year-old added: "When we got there I explained to the police what we were doing. They stayed about two minutes and left because they could clearly see weren't a threat."
Mr Ottaway said he was "sensitive about security" and called the police in adherence to Parliamentary guidelines.
He said: "If you want to have a surgery, you make an appointment.
"One person had made an appointment and I discovered by accident in the afternoon that she had invited an unknown number of people to join.
"We are quite sensitive about security nowadays.
A threat? Campaigners outside the MP's office
"A couple of my Parliamentary colleagues have survived attempted murders at their surgeries and we don't give appointments to people if we don't know who they are. It's as simple as that.
"I let the person who had made the appointment in and the rest stayed outside.
"I notified the police as per the guidance on security for MPs. It is a perfectly straightforward security thing. Any one of those people can make an appointment to see me."
He added: "If I discover than 10 unknown people are to descend on my surgery [again], I will be notifying the police in the future."
The MP said campaigners had issued an "open invition" to protest outside his office and had refused to supply attendees' names.
But campaigners criticised the MP, accusing him of intimidation.
Ms Marsden, who delivered the petition for campaign group 38 Degrees, said: "Everyone was surprised and some of them felt quite intimidated by his [Mr Ottaway's] manner. We were more intimidated by him than he needed to be of us.
"I can see his point, because he was saying he had a colleague who was murdered and two colleagues attacked in their surgeries, so I appreciate his concerns.
Liz Marsden delivers the petition to Richard Ottaway
"But he could see we were not very threatening and the point is they were all his constituents."
The Metropolitan Police confirmed it received a call from Mr Ottaway's office on Friday afternoon.
A spokesman said: "We were called to the office in Brighton Road to reports of a group of demonstrators outside the office.
"Our officers spoke to everyone there but there were no offences being committed so we left."
In 2010, a 21-year-old student was jailed for life for stabbing Labour MP Stephen Timms in an attempt to kill him in his constituency office. She had made an appointment to see him.
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