The Foreign Office minister Ben Bradshaw was sprayed with fake blood on Friday night by an anti-war protester.
Mr Bradshaw was said to be "shaken" but unhurt by the attack at a public meeting on the Afghan campaign in his Exeter constituency. He carried on with the meeting after a man was led away by police.
An organiser said afterwards that the incident raised questions about the lack of Special Branch police protection given to junior ministers at a time of war. Ian Martin, the convener, said the South Street Baptist Church meeting had been under way for about 30 minutes when a man, sitting 15ft from the minister, stood and challenged him.
After shouting at Mr Bradshaw, the attacker raised a plastic bottle and doused him with fake blood, covering his face, hair, shirt and jacket. A Baptist minister and Quaker leader sitting on either side of the minister were also hit, Mr Martin said.
"He looked a little shocked but then carried on," he added. "However, I'm sure it went through his head that it could have been a gun."
Junior ministers are not given Special Branch protection, and Mr Martin said that only two community constables were on hand. The Foreign Office later confirmed the report.
Mr Bradshaw, a former BBC journalist who was first elected to Parliament in 1997, said later: "This was a friendly and very civilised meeting, as one would have expected of a meeting organised by the Quakers. It is a pity one or two people did not approach the meeting in the same spirit of dialogue."
Mr Martin asked: "Why is someone in such an exposed position not given protection? There was a shocked silence and one comment made to me was, 'It could have been a gun'."
Everyone at the meeting – on both sides of the argument – was impressed by the way Mr Bradshaw acquitted himself before and after the incident, according to Mr Martin, himself a Quaker who opposes the Government's policy.
But he went on to say: "I would question whether Ben, even in his own constituency where he is very highly regarded, would speak again without protection."
A spokesman for Devon and Cornwall police said that a man was being questioned at Heavitree Road police station in Exeter about the incident.
The spokesman said that Mr Bradshaw was at a question-and-answer session on the Afghan war when someone threw what turned out to be theatrical blood diluted with water. Some of the liquid went over Mr Bradshaw, as well as the chairman of the meeting.