Russia sent tanks and hundreds of troops into Georgia on Friday after Tbilisi launched ground and air strikes in a major operation to seize control over the rebel region, devastating the province's main city, and killing around 1,500 civilians according to Russian reports.
Saakashvili told the national Security Council in a televised statement: "Georgia is now being subjected to Russian military aggression."
The ongoing conflict is the most severe since South Ossetia fought its way to independence from Georgia in 1992. The majority of the local population have Russian citizenship.
Georgian Interior Ministry spokesman Shota Utiashvili said earlier that Russian combat aircraft had bombed several Georgian military bases, one near the capital Tbilisi, as well as the Black Sea port city of Poti.
Georgian media also reported airstrikes on the city of Gori, and said several civilians had been killed.
However, Russian Deputy Air Force Commander Col. Gen. Anatoly Nogovitsyn denied that warplanes had struck non-military targets.
"We are not fighting peaceful towns, and are not conducting military strikes against civilians. We are only seeking to ensure peace," he said.
Russian President Dmitry Medvedev said earlier on Saturday that the country's troops had begun a military operation in South Ossetia to force Georgian troops to cease violence.
Russia's peacekeeping command said later that the country's troops had driven Georgian forces from South Ossetia's capital, Tskhinvali. Paratroopers from Russia's Ivanovo, Moscow and Pskov airborne divisions have been sent to the city.
Georgia says it has shot down a total of 10 Russian combat aircraft, while Russia says it has lost two planes.
Georgia's Interior Ministry said it was questioning the pilot of one of the Russian planes.
Nogovitsyn told a RIA Novosti news conference that Russian peacekeepers had lost contact with an Su-25 Frogfoot close-support aircraft and a Tu-22 Blinder bomber, and that the fate of the pilots was unknown.
The Russian government has warned that a humanitarian disaster is developing as South Ossetians, many of them injured, flee across the border into Russia. In the past 36 hours around 30,000 people have arrived in Russia.
President Medvedev sent Emergency Situations Minister Sergei Shoigu to Vladikavkaz, where injured refugees are being treated, to coordinate relief efforts. The country has pledged to provide assistance to all refugees from the province.