TBILISI, Georgia — Tens of thousands of Georgians protested the election victory of American-allied President Saakashvili yesterday, claiming fraud and demanding a recount.

The massive demonstration raised fears of instability in the former Soviet republic, which sits on a pipeline carrying Caspian oil to Western markets and has been battleground for influence between Russia and the United States.

It was a dramatic turnaround for Mr. Saakashvili, who rose to power as the hero of the 2003 Rose Revolution protests against fraudulent elections. He has since faced accusations of authoritarian leanings, and his popularity has fallen.

Wearing the opposition’s trademark white scarves, the protesters marched for several hours across downtown Tbilisi in freezing weather to demand a recount of the January 5 election. Organizers said about 100,000 people turned out.

“Misha the Rose, you will fall soon!” protesters chanted, calling the president by his nickname.

Mr. Saakashvili won the election with 53% of the vote, while his main challenger, Levan Gachechiladze, had just under 26%, according to final official results released yesterday.

Mr. Gachechiladze and his supporters accused the government of rigging the vote and demanded that those responsible be prosecuted. They claimed Mr. Saakashvili fell far short of an outright majority and should face off against Mr. Gachechiladze in a runoff.

“Georgia doesn’t have a legitimate president,” Mr. Gachechiladze said at the demonstration. “If we stand together, we will win.”

Opposition leaders also demanded regular access to state television, which has focused on covering Mr. Saakashvili and his allies.

Observers from the Organization for Security and Cooperation in Europe gave a mixed assessment of the election, calling it “triumphant step” for democracy in Georgia while pointing to an array of violations.

Demonstrators yesterday carried signs reading: “OSCE backs rigged elections” and “U.S.A. supporter of dictatorship.”

Mr. Saakashvili, 40, has helped transform Georgia into a country with a growing economy and aspirations of joining the European Union and NATO, cultivating close ties with America and seeking to decrease Russia’s influence.

But a brutal police crackdown on an opposition rally in Tbilisi on November 7 provoked widespread public anger and drew harsh criticism from Western governments. Mr. Saakashvili called the early presidential vote to assuage tensions.

“The November 7 police action against peaceful civilians was outrageous, and official fraud in the presidential vote was disgusting,” a literature expert at the protest march, Irina Berishvili, 52, said. The leader of the Conservative Party, Zviad Dzidziguri, said the opposition alliance would stage regular protests outside state television and other official buildings.

“We will seek to achieve our goals by exclusively peaceful methods,” he said at the rally. “We will win because we defend the truth.”