The dossier, based on media reports, says US-led forces were responsible for more than a third of the deaths.
The survey was carried out by the UK-based Iraq Body Count and Oxford Research Group - which includes academics and peace activists.
The Iraqi government criticised their conclusions, saying Iraqis were most at risk from terrorists who target them.
The Dossier on Civilian Casualties in Iraq 2003-2005 says 37% of all non-combatant deaths were caused by the US-led coalition.
Most of these occurred during the invasion phase, which it counts as ending on 1 May 2003.
But killings by anti-occupation and criminal elements also increased steadily over the entire two-year period.
Insurgents are said to have caused 9% of the deaths, while post-invasion criminal violence was responsible for another 36%.
Targets The number of civilians who have died has almost doubled in the second year from the first, according to the report.
Almost a fifth of the 24,865 deaths were women or children and nearly half of all the civilian deaths were reported in the capital Baghdad.
"On average, 34 ordinary Iraqis have met violent deaths every day since the invasion of March 2003," said John Sloboda, one of the authors of the report.
"The ever-mounting Iraqi death toll is the forgotten cost of the decision to go to war in Iraq," he added.
Mr Sloboda also said: "It remains a matter of the gravest concern that, nearly two-and-a-half years on, neither the US nor the UK governments have begun to systematically measure the impact of their actions in terms of human lives destroyed."
The Iraqi government welcomed the attention the report gave to Iraqi victims, but said it was a mistake to claim that the "plague of terrorism" had killed fewer Iraqis than the multinational forces.
"The international forces try to avoid civilian casualties, whereas the terrorists target civilians and try to kill as many of them as they can," it said in a statement.
"The root cause of Iraq's suffering is terrorism, inherited from Saddam's fascist regime and from mistaken fundamentalist ideology.
"Everybody knows that international forces are necessary in Iraq, on a temporary basis and they will leave Iraq at a time chosen by Iraqis, not in response to terrorist pressure."
'Failure'
The IBC wants to see an independent commission set up in Iraq to give the best estimate of civilian deaths and full details of how each person died.
Human rights groups say the occupying powers in Iraq have failed in their duty to catalogue the deaths of civilians. But the US and Britain say the chaos of war-torn Iraq has made it impossible to get accurate information.
More than 1,700 US soldiers and dozens of other coalition troops are known to have died.
The Iraqi government says 1,300 Iraqi police and military have been killed since security forces were set up in late 2003. But US think-tank the Brookings Institute puts the figure at almost twice this number.
More than half of all civilian deaths were said to have been caused by explosive devices, which disproportionately affected children.
At least 42,500 civilians were reported to have been injured.
The UK-based Iraq Body Count - run by academics and peace activists - is one of the most widely-quoted sources of information on the civilian death toll in Iraq.
The Oxford Research Group describes itself as an independent organisation "which seeks to develop effective methods whereby people can bring about positive change on issues of national and international security by non-violent means".
