The truce was signed as Russia threatened Poland with military action over its deal with the US to install an anti-missile shield.
General Anatoly Nogovitsyn, Russia's deputy chief of staff, said: "Poland, by deploying (the system] is exposing itself to a strike – 100 per cent."
Despite the ceasefire deal first being announced on Wednesday, Russian forces have continued to sweep through Georgian territory, pushing beyond the contested regions of South Ossetia and Abkhazia.
At a joint news conference with Mr Saakashvili, Ms Rice said:
"Our most urgent task is the immediate and orderly withdrawal of Russian armed forces and the return of those forces to Russia. Russian forces need to leave Georgia at once. This is no longer 1968."
Earlier, George Bush, the US president, also demanded Russian forces leave Georgia and accused Moscow of "bullying and intimidation" against the former Soviet republic.
Mr Saakashvili had delayed signing the peace deal, fearing the hastily drafted truce might give the Russians a get-out clause to continue their action.
The most disputed point in the plan was article five, which allows Russian forces to "implement additional security measures" in Georgia pending the establishment of an "international mechanism" to monitor the conflict zone. The wording could be used to justify incursions such as those seen on Wednesday and Thursday in the city of Gori and other Georgian locations.
However, Ms Rice insisted the deal protects Georgia and the phrasing which allows Russia to take additional security measures refers only to "peacekeeping" troops based in South Ossetia and Abkhazia since separatist wars in the early 1990s.
Mr Saakashvili, meanwhile, denounced Moscow's actions, and said his country would never be reconciled to losing any territory to Russia.
He claimed many areas of Georgia remained under foreign military occupation and said his country was "looking evil directly in the eye".
The Georgian president added that his warnings of the Russian threat had gone unheeded and blamed the West for failing to react strongly enough to previous Russian military moves and not granting his country membership of Nato.
Analysis - Home truths about the new world order – because ignorance is no defence
ONE week ago, most Britons thought about Georgia the way Neville Chamberlain thought about Czechoslovakia – "a faraway land of which we know little".
We now know different.
We know that this tiny Caucasian country is a flashpoint in what all sides are calling the New Cold War – the resumption of the 20th-century confrontation between the United States and Russia.
We know also that this war has more than one front: on Thursday, the Americans opened a new one in Poland, agreeing with the Poles to site an anti-ballistic missile system on Polish soil.
The Russian reaction was immediate and fierce, with the generals warning the Poles that their action made it more likely that Russia would target Poland with its missiles. Polish leaders did not worry about the threats.
After centuries of fighting with the Russians, they have come to believe that the only security to be had is to carry a big stick – hence their successful request for the US to add a battery of Patriot anti-air missiles to the package to provide the air defence the Georgians never had. We know a third front is shortly to open, as Ukraine tussles with Russia over Sebastopol.
Although part of Ukraine, the port is leased to Russia for its Black Sea Fleet until 2017. Ukraine controls the lighthouses that guide the ships back to port and has threatened to turn off the lights – literally – if Russia uses its ships to bombard Ukraine's ally Georgia.
We know that this New Cold War is not just about territory, but is also about oil and gas.
Georgia sits astride the only oil pipeline – and soon, the only gas pipeline – linking the West to Central Asia's huge reserves of both.
Cut the pipes, and Russia will be in a position to squeeze Europe, threatening to turn off the gas as it did in 2005 with Ukraine.
As long as the West controls these pipelines, Russia loses its most potent weapon against the West.
This battle extends beyond Europe: Russia and the US are arguing over rights to Arctic oil reserves too.
And we know that even if the shooting stops, Georgia will remain a potential flashpoint because the crucial issue of the enclaves of Abkhazia and South Ossetia has not been settled.
Russia wants the enclaves to be independent. Georgia says they must remain inside Georgian territory.
International law is no help at all because it says that all peoples should be free, but also that a state should have control over all its territory.
We know, also, that the promise of a friendlier Moscow under the smiling new president Dmitry Medvedev, was a fiction.
In fact, it is the former president Vladimir Putin who has clearly shown that he still wears the trousers.
We know that Russia's key foreign policy objective is to dominate, one way or another, the 15 neighbouring states that it once controlled when it was the centre of the old Soviet Union.
And we also know that each superpower will mould its arguments to suit itself.
Russia supports the right of South Ossetia and Abkhazia to be independent, but when Chechnya tried the same thing a few years ago it flattened the capital, Grozny, to deny them.
The US opposes independence for the two Georgian enclaves, but earlier this year supported independence for Kosovo from Serbia.
Finally, and most importantly, we know that in an ever more hostile and complicated world ignorance is no defence.
Cameron set to upstage Brown with Georgia trip
DAVID Cameron, the Conservative leader, will travel to Georgia today after impressing its government with his remarks that Russia was a bully.
He will delay his holiday to Turkey by a day for the visit, organised at the invitation of the Georgian embassy in London.
He will meet Georgia's prime minister, Vladimer "Lado" Gurgenidze, who holds dual Georgian/UK citizenship.
The high-profile mission threatens to upstage Gordon Brown, the Prime Minister, who last night, within an hour of Mr Cameron's visit being made public, swiftly urged Russia to withdraw from Georgia.
Downing Street said Mr Brown spoke by telephone to Mikheil Saakashvili, following the Georgian president's meeting in Tbilisi with the US secretary of state, Condoleezza Rice.
"The Prime Minister described Russia's incursion into Georgia as a completely unjustified violation of Georgia's territorial integrity," Mr Brown's spokesman said. "Russian forces must withdraw immediately."
Mr Brown has also discussed the crisis in the region with the US president, George Bush, the UN secretary-general, Ban Ki Moon, and Russia's president, Dmitry Medvedev.
Aides of Mr Cameron – who earlier this week called Russia a "dangerous bully" – admitted he had not had an invitation from the Russians to discuss the crisis..