"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.
Ethiopians voted in multi-party
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.
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 peacefu