More than 600 have been displaced during the clashes on Tuesday in towns in the east of the country.
Demonstrators accuse a police unit of carrying out killings and human rights abuses against the Oromo people.
The government has blamed the clashes on a border dispute between the Oromo and their neighbours in the Somali region of Ethiopia.
It says it has now sent in the military to carry out a disarmament exercise.
The conflict has been raging for months but escalated this week in violent confrontations.
In August, the government lifted a 10-month state of emergency imposed following more than two years of anti-government protests.
By Africafrique and agencies