Tens of thousands of Ethiopians marched through the capital Wednesday in a government-organised rally condemning the murder of a group of Ethiopian Christians by Islamic State group militants in Libya.

The official rally appeared to be aimed at channelling public anger sparked by the killings, with a huge crowd beginning to gather shortly after dawn in Addis Ababa's huge Meskel Square.

"Our brothers were murdered, the government must do something," shouted Anteneh Tefera, a young demonstrator. "Their blood is not the blood of animals."

The murders have horrified Ethiopians and sparked global condemnation, including from Pope Francis who expressed his "great distress and sadness".

The IS video, released on Sunday, showed militants in Libya holding captives who they described as "followers of the cross from the enemy Ethiopian Church".

It showed one group of about 12 men being beheaded on a beach and another group of at least 16 being shot in the head in a desert area.

"We gather to commemorate the innocent children of Ethiopia butchered by terrorism and burned to death by xenophobia," said one speaker on the podium, also referring to the wave of violence against immigrants in South Africa.

A large number of Ethiopians leave their country -- Africa's second largest in terms of population with more than 90 million people -- seeking work elsewhere.

Many travel to Libya and other north African nations for jobs, as well as to use it as a stepping stone before risking the dangerous sea crossing to Europe.

Protesters responded by singing, "Enough immigration! Change our country by staying home."

Almost two-thirds of Ethiopians are Christians, the majority of those Orthodox Copts -- who say they have been in the Horn of Africa nation since the first century AD -- as well as large numbers of Protestants.

Islam also has an ancient history in Ethiopia, brought to the country by some of the earliest followers of the Prophet Mohammed, who were sheltered there by the Christian king.

"ISIS doesn't Represent Islam," read one banner held by protestor.