At least 14 people were killed in clashes between different ethnic groups in Gambela, Ethiopia.
The United Nations (UN) and Doctors Without Borders (MSF) offices were also targeted by an angry mob, according to local security sources.
The violence was fired following an NGO car driven by an Ethiopian who ran over and killed two children from the Nuer ethnic group on Friday, at a camp for South Sudanese refugees, sources told AFP.
Ten men and women were killed in a retaliatory attack after a group of refugees attacked Ethiopians living around the camp.
Over the weekend, Ethiopians from the Anuak ethnic group, who are traditionally rivals of the Nuer, marched through the city of Gambela and killed four Nuer Ethiopians.
"People are angry, we want revenge. If the police hadn't got involved, plenty of Nuer would have been killed," Addis Alemayu from the Anuak group said.
"With all these refugees coming from South Sudan, things are only getting worse... this is our land, we were here before them."
However, angry protesters attacked buildings and vehicles belonging to the UN, who are accused of aiding Nuer refugees.
The UN's Stephanie Savariaud said, "Protesters broke open the entry gate to one of our residences and destroyed a vehicle."
Groups also targeted the MSF offices and damaged some of the organisation's vehicles, which forced staff to stay in a city hotel.
Over 300,000 people live in Gambela, with Nuer refugees who fled the conflict in neighbouring South Sudan being the majority.
Violence frequently occurs because of other ethnic groups, which makes up the balance of the demographic.