Ethiopia's main opposition party is demanding the release of its leader and another senior official who it says have been placed under house arrest.
Mr Hailu says he is being kept a prisoner in his own home
The government denied anyone had been detained but said that some opposition figures had been put under surveillance as a precautionary measure.
It said the opposition had reneged on a deal aimed at ending the violence and resolve the dispute over the elections.
At least 26 protesters were shot dead by police in Addis Ababa on Wednesday.
It was the culmination of a week of protests over claims of fraudulent elections last month.
Coalition for Unity and Democracy leader Hailu Shawel and senior official Lidetu Ayalew were reportedly placed under house arrest on Saturday.
Journalists who went to Mr Hailu's home were beaten by police with fists and clubs and had their cameras confiscated, witnesses said.
"I am not allowed to leave my residence, my wife and maid are not allowed to leave and no-one is allowed to visit. I am virtually in prison," said Mr Hailu, according to Reuters news agency.
'Out of control'
Information Minister Bereket Simon said the restrictions had been put in place because the men were a threat to state security.
"Immediately after the rejection of the agreement, the government observed new activities from CUD aimed at inciting more violence," he said.
"For this reason the government has denied CUD leaders the opportunity to communicate."
The CUD denied reneging on the agreement.
"We have stated that we will abide by that agreement, but in order to implement it our members have to be released and our leadership allowed to move around, otherwise we cannot function," said Berhanu Nega, the coalition's vice-chairman.
Prime Minister Meles Zenawi defended Wednesday's security crackdown, saying that though he regretted the loss of life, "things were beginning to get out of control".
"It was prudent to stop this with forceful action," he told Reuters on Friday.
He said he remained ready to take "preventative action" to preserve calm.
Friday's agreement - signed by both the ruling Ethiopian People's Revolutionary Democratic Front (EPRDF) and the two main opposition coalitions - was aimed at ending the violence.
They also agreed to let the electoral board investigate more than 300 complaints in the 547 constituencies.
Three weeks after polling, final election results have not been declared.
According to provisional election results, the EPRDF and its allies have won 320 seats so far, giving it a majority in the 547-member parliament.