Ethiopian Prime Minister Meles Zenawi has ordered a month-long ban on demonstrations, following an election disputed by the opposition.
Voter turnout has been much higher than expected
Mr Zenawi also took control of the security forces in the capital Addis Ababa, in moves he said were meant to ensure post-election stability.
There was an overwhelming turnout in Sunday's election, with voters queuing for hours and polling extended.
Foreign observers said they could not verify opposition claims of rigging.
The chief European Union election monitor, Ana Gomez, called the election a victory for democracy, and a credit to all parties who had participated.
She told the BBC it was a bit absurd for the opposition - who have alleged fraud and intimidation - to dismiss the poll at such an early stage.
The main opposition party, the Coalition for Unity and Democracy (CUD) complained of alleged election fraud and mass arrests of their election observers.
"There is a very, very high possibility for my party to reject the results," said CUD chairman Hailu Shawel.
'Peaceful and democratic'
Mr Meles said he would accept an opposition victory if this was confirmed by international observers.
But his ruling Ethiopian People's Revolutionary Democratic Front (EPRDF) seems likely to be returned for a third term.
"I have heard the comments of the foreign observers and the elections were peaceful and democratic," Mr Meles said in an address on state radio.
"As peace should be respected within the city and its environs, the government has decided to bring all the security forces, the police and the local militias, under one command accountable to the prime minister," he said.
Outdoor public gatherings in Addis Ababa were banned "to ensure a violence-free and peaceful atmosphere", he added.
More than 300 foreign observers were allowed into the country for the first time to check the election, regarded as a crucial test of the country's fledgling democracy.
Twenty-six million Ethiopians were registered to vote in the election.
The opposition threat came as millions flocked to polling stations to cast their ballots - and voting was extended for two hours.
The BBC's Mohamed Adow in Ethiopia says that the turnout has been massive by any standard, especially when compared with poor voting figures in the previous election in 2000.
Decision in the balance
Mr Shawel claimed that large-scale voting irregularities had meant that the election could not be regarded as free and fair.
His assessment was echoed by Beyene Petros, vice-chairman of United Ethiopian Democratic Forces (UEDF).
"The situation in Addis Ababa may be okay but not outside," Mr Beyene told AFP. "In much of the countryside, we don't think we'll accept the results of the polls, but we haven't made our final statement yet."