Thousands of people have been arrested across Ethiopia (search) following violent clashes in which police killed 36 people, a U.S.-based human rights group reported Wednesday.

Human Rights Watch (search) said student activists and opposition supporters had been rounded up in a crackdown after last week's fighting.

"Opposition rhetoric may well have contributed to last week's unrest, but the government must take responsibility for the conduct of its own security forces," said Georgette Gagnon, deputy Africa director at Human Rights Watch. "The security forces have killed dozens of protesters and arbitrarily detained thousands of people across the country."

Ethiopian federal police said some detainees were being held at Ziway detention facility, 90 miles south of the capital, but could not give exact numbers.

"I don't have the exact figure but some detainees are in Ziway because of these disturbances," deputy police commissioner Hassan Shauffa told The Associated Press. "I don't think there are hundreds let alone thousands arrested in the regions."

Arrests have continued in the capital, where police detained at least three members of the Ethiopian Human Rights Council earlier this week. New York-based Human Rights Watch said it had obtained reports of mass arrests in at least nine cities outside the capital since last Monday.

"While international attention has focused on events in Addis Ababa, opposition members and students in other cities are increasingly at risk of arbitrary arrest and torture," added Gagnon.

She said many of the people arrested in earlier round-ups had been released "but smaller-scale arrests targeting CUD supporters and student activists have continued unabated."

Ethiopia's main opposition leader was freed from house arrest after the country's main political parties agreed to work together for peace after 10 days of political unrest left at least 37 people dead.

Hailu Shawel, the leader of the opposition Coalition for Unity and Democracy (search), told The Associated Press he was informed Tuesday that his house arrest was lifted after two days of talks mediated by the European Union.

"This is a breakthrough and should now get the political process back on track," said European Commission Ambassador Tim Clarke, who mediated the talks.

The agreement lays the groundwork for all political parties to play a role in the investigations of complaints over the elections. Although the three main political parties signed up to a nonviolence pact on Friday, the deal foundered after the opposition imposed conditions they have now retracted.

But tensions still remain and political parties are expected to meet Wednesday to continue discussions on improving relations.

Ethiopia's ruling party, which has pledged itself to democratic reform but shown authoritarian tendencies, claimed victory in last month's elections based on provisional results. But parties have lodged complaints in 299 of the 527 constituencies in the country.

The violence that has followed threatens to destabilize Ethiopia, one of the poorest countries in the world, which faces cyclical drought and widespread hunger. It also could strain Meles' dealings with the international community.

In Washington, the State Department called on the Ethiopian security forces Tuesday to avoid use of excessive force in dealing with postelection violence. Spokesman Sean McCormack also urged students, civil society, opposition supporters, government party members and political leaders to exercise restraint as well.

"We condemn election-related violence in Ethiopia," McCormack said.