YEREVAN, Armenia - Thousands of opposition supporters marched through Armenia's capital Wednesday after an election official said complete results showed that the prime minister had won the presidential election.

Allegations of fraud and threats of mass protests have raised concerns about the stability of the volatile, strategic country, located at the juncture of the energy-rich Caspian Sea region and southern Europe and bordering Iran.

An initial count of the ballots showed Prime Minister Serge Sarkisian had nearly 53 percent of the vote in Tuesday's election, Central Election Commission chief Garegin Azarian said -- enough to win outright and avoid a runoff. Top opposition candidate Levon Ter-Petrosian had 21.5 percent, Azarian said.

Ter-Petrosian's backers have alleged widespread fraud, and a crowd gathered in central Yerevan to protest the results, swelling to some 20,000 as riot police with truncheons guarded the election commission building a five-minute walk away.

The protesters then marched to the government headquarters in a broad central square, many shouting "Levon!" and raising a clenched fist -- Ter-Petrosian's campaign symbol. Helmeted police blocked the building, and the protesters moved on, marching toward the election commission building.

"I have no doubt that the authorities have falsified the election and I will protest with all those who also feel cheated," Simon Grigorian, a 38-year-old engineer, said at the protest.

Sarkisian was groomed by outgoing President Robert Kocharian and was widely expected to win, in part because of favorable media coverage and support from the state bureaucracy. He also has benefited from economic improvements in recent years.

Ter-Petrosian accused the authorities of resorting to ballot-stuffing, vote-buying and beating his activists who monitored the election. He asserted that he was actually the winner.

"These figures have nothing to do with reality, we are overwhelmingly ahead of them," his spokesman, Arman Museian, said of the results announced by the election commission.

An influential international observer mission said there were concerns about the vote count, but issued a generally positive assessment of the election.

The election was "mostly in line with the country's international commitments, although further improvements are necessary," the mission from the Organization for Security and Cooperation in Europe said in a preliminary statement.

"The problems we observed, particularly during the counting, must be addressed to increase overall confidence in the electoral process," Anne-Marie Lizin, one of the mission's coordinators, said in the statement. The observers declined to comment on whether the problems could have significantly affected the outcome of the vote.

A candidate needed to garner more than 50 percent of the vote to avoid a runoff.

Sarkisian and Ter-Petrosian -- Armenia's first president after the 1991 Soviet collapse -- were the two top contenders vying to lead the South Caucasus nation, where more than a quarter of its 3.2 million people live in poverty despite some economic progress in recent years.

The election campaign was dominated by the country's economic revival and efforts to resolve the status of Nagorno-Karabakh -- a mountainous region in neighboring Azerbaijan that has been under ethnic Armenian control since a cease-fire ended six years of fighting in 1994.

Kosovo's declaration of independence from Serbia on Sunday added an element of uncertainty for Armenians, many of whom see clear analogies between Kosovo and Nagorno-Karabakh.

The Armenian government says the region should be recognized as a sovereign state, while Azerbaijan says it will never cede its territory.

Armenia experienced an economic collapse after the Soviet breakup and in the face of blockades by Azerbaijan and its key ally Turkey, which is outraged by efforts to win international recognition of the killing of 1.5 million Armenians by Ottoman Turks in the World War I-era as genocide.

The blockades have hamstrung Armenia's economy and cut it out of lucrative regional energy and transport projects.

The U.S., whose large Armenian diaspora has a strong lobby in Congress, has poured some $1.7 billion in aid into the country since 1991, encouraging economic and political liberalization.

Armenia is eligible for millions more in U.S. aid, but a questionable election could jeopardize Washington's support.