"Security forces have been instructed to take severe actions against those who violate the new regulation," Prime Minister Meles Zenawi said in a televised address on Sunday, adding he had put himself in charge of all security forces in the capital.
"The step was taken to protect the nation from elements who want to foment trouble."
Millions voted in the parliamentary poll, only the second real multi-party contest in Africa's top coffee producer, which is expected to hand a third term to Meles.
"Those who may have reservations over the election results are expected to air their grievances through the constitutional processes of the country. Anything that violates the constitution will not be tolerated," Meles said.
European Union observers said they had no immediate word of major irregularities, but opposition groups dismissed that assessment, saying there had been ballot-rigging by government supporters in remote areas.
Opposition parties advocating a bigger role for free markets in the impoverished country of 72 million have set aside their ethnic differences for the first time, pledging to unite if it means winning a majority in the 547-seat national assembly.
Ethiopians voted in multi-party
ballots, the second time in history
But political analysts say a victory over Meles' dominant Ethiopian People's Revolutionary Democratic Front (EPRDF) is unlikely, given that the opposition currently has just 20 seats.
"There have been fears of impending violence among people for some time, as a result of negative attitudes by some people who took part in the election," said Meles who toppled dictator Mengistu Haile Mariam in 1991 to end 17 years of Marxist rule.
His remarks provided an abrupt change of tone at the end of a day marked by an apparently peaceful display of voting by large numbers among Ethiopia's 26 million registered voters.
The stream of voters swelled through the morning at polling stations in Addis Ababa's mist-shrouded hills, as church services ended and worshippers turned up to vote.
Voters in the capital's Gullele sub-district cheered as a bride and groom, resplendent in wedding dress, arrived to vote.
Student Tirsit Kaffa said: "We are happy. It's the first time for democracy in Ethiopia and we are voting for change."
In Dukem village, 35km south of the capital, Kedja, an illiterate 50-year-old peasant woman, in traditional white shawl and robe, said she knew nothing of the opposition.
"I will vote for the government," she said, lining up at a tin-roofed mud building serving as a polling station. "I am not aware of any opposition parties. I just know the government."
Bereft of experience at the polls and subject to what they call harassment by ruling party cadres, opposition candidates have yet to mobilise grassroots support in rural areas, which is critical to their success, diplomats and political analysts say.
Millions of Ethiopians voted in
the elections on Sunday
More than 300 foreigners are observing the polls, the most ever allowed at an Ethiopian election. Former US president Jimmy Carter is heading one of the teams.
Chief EU observer Ana Gomes said that in eastern Harar town, voters had complained of intimidation, saying the chairman of their polling station said they were to vote for the bee, the EPRDF's election symbol.
In another area, police officers were seen controlling the crowd with sticks and guns, while people with machetes were allowed into the polling station, she added.
EU observers, however, found no evidence of opposition allegations that many of their observers had been detained.
"Our preliminary assessment is there is no public disorder. People are very keen and motivated to vote; they think it's different this time," Gomes said.
Beyene Petros, vice-chairman of the United Democratic Ethiopian Forces coalition, strongly disagreed.
"The EU observers have not gone deep into the regions to observe ballot rigging and irregularities perpetrated by government supporters," he said.
Hailu Shawel, chairman of the opposition Coalition for Unity and Democracy, said: "The situation is completely disastrous.
"High fraud is taking place in many polling stations in Addis Ababa and the regions. We are not happy with the assessment of the EU observers. They appear biased against the opposition."
Official results are due on 8 June.