Tuesday's rally took place hours before Russia and Georgia agreed in principle on a peace plan, and after Russia ended its military operation to prevent further Georgian incursions into breakaway South Ossetia.
"We came to fight because a northern neighbor wants to suppress a small country. We want to tell it 'No!'," Polish President Lech Kaczynski said.
"This country thinks that old times will come back, but those times are over. Everyone knows that Ukraine, then Poland could be next," Kaczynski said.
The presidents of Ukraine and the three post-Soviet Baltic countries - Lithuania, Latvia and Estonia - were also on the podium with Georgian leader Mikheil Saakashvili.
The rally came after five days of fighting that began with an attack by Georgian forces on the South Ossetian capital of Tskhinvali on August 8. Russia has said that around 1,600 people died in the Georgian assault. Some 34,000 people also fled fighting into Russia. Most residents of South Ossetia have Russian citizenship.
Russia has accused Ukraine of supplying arms to Georgia and encouraging it to attack South Ossetia.
Ukrainian President Viktor Yushchenko told the crowd in Tbilisi that his country will support Georgia in its battle for independence.
"If they put under question democracy and independence in Georgia, then they can undermine democracy and independence in other states," he said.
Latvia sent its prime minister to the rally, rather than President Valdis Zatlers, who is seen to have a more accommodating attitude to Russia.
Premier Ivars Godmanis said: "Always be united, and we - Latvians, Lithuanians, Estonians, Poles, and Ukrainians - will always be with you,"
Latvia has joined the United States in warning that relations with Russia are bound to suffer following the conflict.
The presidents of Russia and France agreed on Tuesday on six principles to resolve the situation in South Ossetia.
"The first is not to resort to the use of force. The second is to halt all military action. The third is free access to humanitarian aid. The fourth is that Georgian Armed Forces should return to their bases. The fifth is that Russian Armed Forces should pull back to their positions prior to combat," Russian President Dmitry Medvedev told a news conference with his French counterpart, Nicolas Sarkozy.
