With awithin shooting distance of Russian troops, Medvedev's comments were another reminder that the Kremlin views last month's war as the start of a new era in Russian assertiveness.
Prime Minister Vladimir Putin, meanwhile, said "the truth is on our side" and likened the situation in the breakaway Georgian region of South Ossetia with Srebrenica - the Bosnian town that was the site of Europe's worst mass carnage since World War II.
In France, the European Union's 27 foreign ministers were reluctant to provoke Moscow, with French Foreign Minister Bernard Kouchner saying the EU did not plan to impose sanctions against Russia.
"Russia must remain a partner, it's our neighbor, it's a large country and there is no question to go back to a Cold War situation, that would be a big mistake," Kouchner said.
In the weeks since Russian forces routed the Georgian army and seized the breakaway Georgian region of South Ossetia, Russian officials have used bellicose language toward the West. Putin has suggested the United States was to blame for the war for helping the Georgian military rebuild.
At a meeting Saturday of the State Council, Medvedev said the world had changed since the beginning of fighting in Georgia last month.
"We have reached a moment of truth. It became a different world after Aug. 8," he said.
"Russia will never allow anyone to infringe upon the lives and dignity of its citizens. Russia is a nation to be reckoned with from now on," Medvedev told the council, a government consultative body of largely regional governors.
Medvedev criticized the United States and other Western nations, though not by name, for challenging Russia's intervention.
"Millions of people supported us, but we've heard no words of support and understanding from those who in the same circumstances pontificate about free elections and national dignity and the need to use force to punish an aggressor," he said.
The United States has moved to counter Russia, both lambasting Moscow for what it called a disproportionate military response and providing humanitarian and economic aid to Georgia.
U.S. Vice President Dick Cheney, at an economic meeting Saturday in Italy, blasted Russian actions in the war as an "affront to civilized standards" and said Moscow has given "no satisfactory justification" for invading Georgia.
During a visit to North Africa, U.S. Secretary of State Condoleezza Rice said that "the time isn't right" for the U.S. to move forward on a once-celebrated deal for civilian nuclear cooperation with Russia, Secretary of State Condoleezza Rice said Saturday.
Her comment increased speculation that President Bush is, a former Soviet republic, by canceling the agreement. Such a move is being planned, according to senior Bush administration officials, but is not yet final.
U.S. warships have delivered much of the aid and Russian officials have questioned whether the aid is a cover for weapons shipments.
At Georgia's Black Sea port of Poti, Russian forces watched closely Saturday as the U.S. naval ship USS Mount Whitney delivered 17 tons of aid for Georgians displaced by the fighting.
U.S. naval officers said a Russian warship had trailed the Mount Whitney - the flagship of the U.S. Navy's Mediterranean fleet - across the Black Sea. Russian forces onshore were also scrutinizing the ship from a position just 3 miles away from its anchorage off Poti.
"They're clearly watching us very, very closely, and I think they'll be very happy when we leave," the ship's commanding officer, Capt. Owen Honors, told The Associated Press.
Capt. John Moore, the commander of the task force that has brought some 450 tons of aid to Georgia on three U.S. ships and numerous planes, said the Russian frigate Ladnyy had trailed the Whitney about 4,000 yards away for the entire Black Sea trip. The Russian boat remained in international waters after the U.S. ship crossed Friday into Georgian waters 12 miles from Poti, he said.
In an echo of the cat-and-mouse games that Soviet and American forces played in the Cold War, Moore said the two countries' naval forces had had little contact except for a brief exchange between the Ladnyy and another U.S. ship, the USS McFaul.
"I think it was on the 24th August, when the Ladnyy contacted the bridge and very courteously said 'Hey, welcome to the Black Sea', and we responded in kind 'thank you very much'," he told AP.
At one Russian position on the shore near Poti, several light tanks and armored personnel carriers bearing peacekeeping insignias could be seen Saturday behind a high earthen berm and a razor-wire fence. An excavator dug new holes nearby.
These dug-in Russian troops were still on Georgian territory weeks after an EU peace deal required them to leave. Soldiers refused to let an Associated Press reporter enter the post and said the commander was not there, but an officer acknowledged the Russians had seen the ship.
Georgia, a South Caucasus nation long dominated by Russia, sits astride a strategic corridor for Caspian Sea and Central Asian oil and gas. Georgia's desire to join NATO and move closer to the West has angered Russia.
Since the war, Russia has recognized South Ossetia and another separatist province, Abkhazia, as independent nations despite protests from the European Union, the United States and Georgia.
Putin, who often appears to taunt the West, insisted in an interview broadcast late Saturday, that Russia was justified in its intervention in South Ossetia. He said there would be no cooling of ties with the West because the West is dependent Russia's oil, gas and mineral wealth.
"We are convinced that the truth is on our side," he said in the interview with state-run TV.
He also drew parallels between South Ossetia and Srebrenica, the town where Serb troops in 1995 killed some 8,000 Muslim men and boys. He said the European peacekeepers there at the time - mainly Dutch soldiers, operating under a U.N. mandate - stood aside as the massacre took place.
In the French city of Avignon, EU foreign ministers met to figure out how the bloc can mediate a long-term solution to the standoff. Kouchner insisted the EU's aim was also to improve relations with Russia, despite current disagreements.
French President Nicolas Sarkozy was heading to Russia on Monday to meet with Medvedev and clarify parts of the EU peace deal, especially the terms for withdrawing troops.