American forces were responsible for more than a third of those deaths, while criminals accounted for a similar number and anti-US forces carried out about 10% of the killings, the Iraq Body Count research project, published on Tuesday, found.
"The ever-mounting Iraqi death toll is the forgotten cost of the decision to go to war in Iraq," said John Sloboda, a psychology professor at Keele University in central England and co-founder of Iraq Body Count.
"On average, 34 ordinary Iraqis have met violent deaths every day since the invasion of March 2003."
Britain's Foreign Office said it did not have its own figure for civilian fatalities in Iraq.
"There are no wholly reliable figures for civilian deaths," a spokesman said. "It is recognised by everybody that statistics are very hard to collect under these circumstances.
"Any civilian being killed in war is a terrible thing," the spokesman added.
The Iraq Body Count estimate was much lower than the figure of 98,000 civilian deaths that appeared in a study in medical journal The Lancet in October 2004.
Iraq Body Count compiled the figure of 24,865 civilian killings occurring between 20 March 2003 and 19 March 2005 from media reports.
It relied mostly on online English-language reports by the major news agencies and British and American newspapers.
"The ever-mounting Iraqi death toll is the forgotten cost of the decision to go to war in Iraq"
John Sloboda, professor,
Keele University, UK
The group found that 7299 civilian killings - or 30% of the total - occurred in the six weeks until 1 May when US President George Bush declared major combat operations over.
Another 6215 civilians died in the period to 30 March, 2004, and 11,315 died in the period to 19 March, 2005.
Foreign troops were responsible for 9270 deaths - or 37.3% of the total - and American troops were accountable for 98.5% of those fatalities.
An additional 11% of the deaths were caused by unknown
agents - attacks that lacked a clearly identifiable military objective.
However, Iraq Body Count said there would likely be some overlap between the "unknown agents category and the
"anti-occupation forces" one.
US-led forces and anti-US fighters were jointly involved in a further 2.5% of the killings.
Iraq Body Count identified 35.9% of the deaths as "predominantly criminal killings", linked to the huge crime wave that struck Iraq after the collapse of Saddam Hussein's government.
These include large numbers of ordinary people killed in robberies and kidnappings, inter-ethnic and inter-religious clashes and turf battles between criminal gangs.
Also, 3.8% of the deaths were blamed on what the Iraqi Ministry of Health described as "military actions" and "terrorist attacks".
It was not immediately clear how Iraq Body Count separated these deaths from those it attributed to US-led or anti-US forces.
The study also showed that the number of deaths per month caused by non US-led forces increased steadily over the two years.
More than half of all the civilian deaths (53%) involved explosive devices, while 34% involved aircraft.
Sloboda said the Iraq Body Count report did not discredit the Lancet study, which was based on a small number of interviews with Iraqis about their experiences of the invasion and occupation.