An investigation into last year's Russia-Georgia war delivered a damning indictment of President Mikheil Saakashvili today, accusing Tbilisi of launching an indiscriminate artillery barrage on the city of Tskhinvali that started the war.

In more than 1,000 pages of analysis, documentation and witness statements, the most exhaustive inquiry into the five-day conflict dismissed Georgian claims that the artillery attack was in response to a Russian invasion, accused both sides of violations of the laws of war, indicated that war crimes had been perpetrated against Georgian civilians and rejected Russian claims of "genocide" in the breakaway Georgian province of South Ossetia.

The EU-commissioned report, by a fact-finding mission of more than 20 political, military, human rights and international law experts led by the Swiss diplomat, Heidi Tagliavini, was unveiled in Brussels today after nine months of work.

"There is no way to assign overall responsibility for the conflict to one side alone," the report found.

But the conclusions will discomfit the western-backed Georgian leader, Saakashvili, who was found to have started the war with the attack on Tskhinvali, the South Ossetian capital, on the night of 7 August last year, through a "penchant for acting in the heat of the moment".

The war started "with a massive Georgian artillery attack", the report said, citing an order from Saakashvili that the offensive was aimed at halting Russian military units moving into South Ossetia.

Flatly dismissing Saakashvili's version, the report said: "There was no ongoing armed attack by Russia before the start of the Georgian operation ... Georgian claims of a large-scale presence of Russian armed forces in South Ossetia prior to the Georgian offensive could not be substantiated ... It could also not be verified that Russia was on the verge of such a major attack."

While concluding the Georgians fired the first shots, the report said the attack was the culmination of months and years of rising tension and provocations for which both sides bore the blame.

Intended to settle intense finger-pointing over the conflict, the report predictably unleashed a fresh bout of charge and counter-charge between the Russians and the Georgians.

The investigators criticised and condemned Russian conduct and policy in the months and yearsleading up to the war and its behaviour since. But on the issues of who started what when, the report was unequivocal. The Georgian offensive against Tskhinvali was not justified under international law.

"It is not possible to accept that the shelling of Tskhinvali with Grad multiple rocket launchers and heavy artillery would satisfy the requirements of having been necessary and proportionate."

The Russians had moved mercenaries and paramilitary forces into South Ossetia in apparent preparation for armed hostilities before Saakashvili's disastrous offensive, which triggered a Russian invasion and left his country partitioned. But the proper Russian reponse to the artillery barrage came – by land, sea and air – 12 hours after the Georgian action.

The report concluded that South Ossetian irregular forces violated the rules of war in attacks on Georgian villages and that Russian peacekeeping forces "would not or could not" control them. Russian claims of Georgian "genocide" in South Ossetia were dismissed and Russian claims that Georgians had killed 2,000 civilians were found to be wildly exaggerated. The report put the figure of civilian dead at 162 on the South Ossetian side.

The secession of South Ossetia and Abkhazia from Georgia was branded illegal and Russian recognition of the two "states" in breach of international law.The report found that Moscow had been assiduously preparing the secession by, among other things, a policy of "passportification", illegally distributing Russian passports on a mass scale among the breakaway populations.The mission also found that western blunders spurred the warmongers. A "series of mistakes, misperceptions, and missed opportunities on all sides" internationally contributed to the breakout of war. It traced the conflict back to the early 90s and the fallout from the collapse of the Soviet Union and accused the Kremlin of abusing its status as a "great power" to coerce "a small and insubordinate neighbour."

The Russian forces in South Ossetia failed to stop irregulars conduct a campaign of ethnic cleansing against Georgian villages, entailing looting, rape, hostage-taking, and arbitrary arrest.

Key excerpts from the report on the five-day Russian-Georgian war:

"On the night of 7-8 August 2008 … heavy fighting erupted in and around … Tskhinvali in South Ossetia [and] soon extended to other parts of Georgia. It caused serious destruction, reaching levels of utter devastation. Altogether about 850 persons lost their lives, more than 100,000 civilians fled their homes. "

"The Russian side justified their military intervention by their intention to stop an allegedly ongoing genocide of the Ossetian population by the Georgian forces, and also to protect Russian citizens residing in South Ossetia and the Russian contingent of the Joint Peacekeeping Forces deployed in South Ossetia."

"The shelling of Tskhinvali by the Georgian armed forces during the night of 7 to 8 August 2008 marked the beginning of the large-scale armed conflict in Georgia, yet it was only the culminating point of a long period of increasing tensions, provocations and incidents. Indeed, the conflict has deep roots in the history of the region, in peoples' national traditions and aspirations as well as in age-old perceptions or rather misperceptions of each other, which were never mended and sometimes exploited."

"There is the question of whether the use of force by Georgia in South Ossetia … was justifiable under international law. It was not [under existing stability accords with Moscow]."

"Georgian claims of a large-scale presence of Russian armed forces in South Ossetia prior to the Georgian offensive … could not be substantiated."

"It seems that much of the Russian military action went far beyond the reasonable limits of defence. This holds true for all kinds of massive and extended action …