British Parliamentary Committee for Iran Freedom chairman Lord Corbett hit out in a strongly-worded open letter.
More than 3,000 people live in the Camp Ashraf, and supporters say several died in a raid by Iraqi security forces.
The Foreign Office says officials will visit the camp soon and have written to Iraq's government over the matter.
Those at the camp, which was previously under US control, are members of an exiled Iranian opposition group, the People's Mujahadeen of Iran (PMOI).
Their case has won the backing of senior Church of England clergy, led by the Archbishop of Canterbury, Dr Rowan Williams.
“ Is this the kind of democracy for which our government sent troops to die and be injured? ”,
Lord Corbett asks.
Lord Corbett wrote: "At Labour's conference you will rightly join the chorus of condemnation at Iran's menacing nuclear deception.
He described it as "a brutal assault on refugees who offered no more than passive resistance against 'security forces' armed with hand-held chains, axes, iron bars and wooden clubs embedded with nails as well as live ammunition".
Lord Corbett claimed this raid, on 28 July, had left 11 dead and more than 500 injured.
"Some 36 were arrested and remain detained despite court orders for their release. Is this the kind of democracy for which our government sent troops to die and be injured?"
He said the UK had a responsibility to help ensure the safety and security of the refugees and called on Mr Miliband to try to ensure a UN monitoring force could go to the camp.
"Speak out this week, David, please," he added.
The Foreign Office said British officials visited the camp in April and June and would visit again soon. Britain's Ambassador in Baghdad had also asked the Iraqi government for a review of events on 28 July.
Embassy officials had met US, UN and Iraqi counterparts to discuss the situation and were also in contact with the UN Commission on Human Rights, a spokesman added.
Iraq said at the time of the raid that it was trying to establish a police station at the camp.