Hailu said he was not allowed to leave his residence, nor was his wife or maid. He said visitors were also barred from entering.
"I am virtually in prison," said Hailu, chairman of the main opposition Coalition for Unity and Democracy (CUD).
Witnesses said journalists who went to interview the politician at home were roughed up and their cameras confiscated. But diplomats have been allowed to see the opposition leader.
Protests began on Monday, defying a ban ordered by Prime Minister Meles Zenawi, after weeks of opposition allegations that the ruling Ethiopian People's Revolutionary Democratic Front (EDRDF) intimidated voters and rigged elections last month.
Violence erupted on Wednesday when police and troops fired into crowds of protesters, killing at least 26 people. Since then, security forces have rounded up youths and detained opposition members.
Meles said on Friday that the government would stop opposition leaders from leaving the country and could detain people without question "if things get hotter".
CUD deputy chairman Berhanu Nega was stopped at the airport while trying to leave for London on a fund-raising trip.
Opposition officials said three other senior CUD officials had been placed under house arrest on Saturday. More than a dozen other members also remain in police custody.
"I am virtually in prison"
Information Minister Bereket Simon said CUD leaders had rejected a deal on Friday which called on the opposition and the ruling party not to incite violence and to support the work of the electoral commission.
He said the CUD had demanded that the government release all those arrested over the violence.
"Immediately after the rejection of the agreement, the government observed new activities from CUD aimed at inciting more violence," Bereket said. "For this reason, the government has denied CUD leaders the opportunity to communicate."
The EPRDF accuses the CUD of stirring the violence in Addis Ababa, and says police opened fire after looters and rioters attacked them with stones. The CUD says the protests were spontaneous.
Witnesses said calm had returned in the capital on Saturday with taxis back on the roads and some shops opening their doors.
Diplomats called last month's election the most democratic in Ethiopia's history.
Prime Minister Meles Zenawi says
the CUD is encouraging violence
But a delay in announcing the official results, now due on 8 July, compounded by claims of victory by both sides and accusations of fraud, have prolonged the tension in Africa's top coffee grower.
Early results show the EPRDF and allies won enough seats to give them a third five-year term ruling the nation of 72 million, sub-Saharan Africa's second most populous state.
But the opposition surprisingly increased its share of parliamentary seats nearly tenfold and made a clean sweep in Addis Ababa.