Ethiopia has started three days of national mourning following the killing by Islamic State of more than 20 migrant workers - most thought to be Ethiopian Christians - in Libya.
The Libyan branch of IS on Sunday released videos of the men being beheaded and another group being shot.
The killings have been condemned in Ethiopia and throughout the world.
IS and other jihadist groups are active in many towns in Libya, which has been torn by civil conflict since last year.
The brother of one of those killed described the killers as "animals... outside of all humanity".
Ethiopia's government has confirmed that the people shown being killed in the IS videos were Ethiopian migrant workers.
However the Jerusalem Post reported that three of those killed were Eritreans who had previously sought asylum in Israel.
Among those who have condemned the killings are Pope Francis, who expressed his "great distress and sadness" and Ethiopian Orthodox Tewahedo Church Patriarch Abune Mathias who described them as "repugnant".
"We have a duty to raise our voice to tell the world that the killing of the innocent like animals is completely unacceptable," he was quoted by the AFP news agency as saying.
Prayers were said jointly by Christian and Muslims across Ethiopia, led by Sheikh Mohammed Jemal, head of the Islamic Affairs Supreme Council. He said that the killing of people like "chickens" had no place in Islam.
A man who saw his only brother executed on the video also expressed his outrage.
"I saw him kneeling, a masked man pointing a gun to my brother and his friend, with a knife to their throats," Tesfaye Wolde said.
Rival groups in Libya
He described how his brother and his brother's friend - also seen killed on the video - left Ethiopia two months ago to seek a better life in Europe, heading first for Sudan and then to Libya in the hope of crossing the Mediterranean to Italy.
Ethiopians have taken to social media to express their shock, anger and grief following the killings.
The African Union, the European Union and the US have also expressed their solidarity with Ethiopia.
The video is similar to previous ones posted by IS, including the beheading of 21 Egyptian Christians in February.
The latest killings appear to have taken place in two regions of Libya - in the south and the east, the BBC's North African correspondent Rana Jawad reports.
Libya has not had a stable government since long-time leader Muammar Gaddafi was toppled in 2011.