Officials in Armenia say the incumbent President, Robert Kocharyan, has won a clear victory in the second round of the presidential election.
President Kocharyan narrowly missed an outright victory in the first round
With nearly all the votes counted, Mr Kocharyan is said to have gained 67% - far ahead of his challenger, Stepan Demirchyan.
But international election monitors said on Thursday there had been widespread ballot stuffing.
Mr Demirchyan has also claimed that the result was rigged and called a protest rally to contest it.
"We are disappointed, we hoped for better," said Peter Eichel the head of the Organisation for Security and Council (OSCE) monitoring group.
I fear that blood will be spilled, and I fear the scenario of Yugoslavia in 2000
Political analyst Mark Grigorian
Lord Russel Johnston, head of the Council of Europe monitoring mission, added: "Our observers witnessed ballot-box stuffing. It was a simple pattern - a good voting day followed by a less then satisfactory counting night."
President Kocharyan narrowly missed outright victory in the first round, falling 0.5% short of the necessary 50%.
'Intimidation and violence'
"We will not be making any official announcements yet... but I
think you can already congratulate us," said an official at Mr Kocharyan's election headquarters.
Mr Demirchyan claimed overnight that there had been "massive legal violations" and condemned what he described as "an environment of intimidation and violence".
He added: "The outcome of this election
has nothing to do with the people's choice."
Political analyst Mark Grigorian said there was a risk of violence, as occurred after a disputed election in 1996.
"I fear that blood will be spilled, and I fear the scenario of Yugoslavia in 2000," he said.
"The extent of ballot box stuffing shows that Kocharyan is absolutely determined to stay in power.
"I do not exclude that he will be willing and quick to use force," Grigoryan said.
Early results gave Mr Demirchyan on 32.5% in the second round, compared to 28.22% officials said he won in the first round two weeks ago.
Tens of thousands of Mr Demirchyan's supporters took to the streets to protest against the first-round results, claiming that their candidate was the rightful winner.
The opposition camp also accused Mr Kocharyan's backers of intimidation after police arrested about 150 Demirchyan supporters for public order offences.
Mr Kocharyan, 49, is credited with bringing a measure of stability to the country in the five years he was in office.
Energy supplies have been widely restored under his leadership, but poverty is widespread, with average monthly salaries of about $40.
His 43-year-old rival, who has run on an anti-corruption platform, is the son of Soviet-era Communist leader Karen Demirchyan, who was shot dead in a 1999 parliamentary massacre.