Russian forces remain in the two breakaway regions
The Organisation for Security and Cooperation in Europe has been accused of failing to warn that this summer's Russia-Georgia conflict was looming.
A former senior OSCE official, Ryan Grist, told the BBC he had warned of Georgia's military activity before its move into the South Ossetia region.
He said it was an "absolute failure" reports were not passed on by bosses.
But OSCE Chairman Alexander Stubb said the risks were transmitted to member governments and the system worked well.
The conflict in the region began on 7 August when Georgia tried to retake its breakaway region of South Ossetia by force after a series of lower-level clashes with Russian-backed rebels.
Russia launched a counter-attack and the Georgian troops were ejected from both South Ossetia and Abkhazia, a second breakaway region, days later.
Russian forces remain in the two regions, and Moscow has backed their declarations of independence.
On Friday, thousands of anti-government activists demonstrated in the Georgian capital, Tbilisi - their first major protest since the conflict.
Mr Grist said: "The OSCE had been working in South Ossetia for many many years. We were the one institution that knew, had a feel for what was going on there at the mission level.
Opposition supporters want a "new wave of civil confrontation"
"There clearly wasn't the eye on the ball on the higher diplomatic level I would say. Because it was clear that something was brewing."
He said he had made it "very clear" at a briefing to ambassadors there was a "severe escalation".
"It would give the Russian Federation any excuse it needed in terms of trying to support its own troops," Mr Grist said.
But Mr Stubb said reports were sent to member governments, who then made their own assessments.
He added: "I myself got worrying information around the 7th of August that something, so to say, is cooking but that was going on all the time on both sides."
Mr Stubb said the OSCE only had "diplomatic means" but admitted those means had failed.
"That's why we had to act immediately when the war had started. Then we only had one aim and that was a ceasefire and I think we succeeded quite well in that."
The OSCE has had a mission in Georgia since 1992 and last month co-hosted talks with the EU and UN that ended without the two protagonists meeting.
In Friday's protests in Tbilisi, opposition leader Kakha Kukava said: "We are starting a new wave of civil confrontation, and we will not give up until new elections are called."
Critics have accused President Mikhail Saakashvili of starting a war that Georgia could not win.
There were about 10,000 protesters at Friday's rally, according to estimates by reporters at the scene.
They crowded the steps outside parliament, holding banners and waving flags, calling for democratic reforms.
They were fewer than the 30,000 the opposition had hoped for, and fewer than at protests a year ago.
The president is adamant Georgia was provoked by Russia, and has called for an inquiry.