"The government has done nothing so far," he said, demanding that the United Nations, United States and European Union "defend the rights of Christians in Mosul".
"We do not want to be involved in the rivalries of political parties running for election," he said, referring to nationwide legislative elections set for March 7.
And Pope Benedict XVI added his voice to the protest, sending a message to Baghdad in which he condemned the violence against Christians in Iraq and asked Iraqi authorities to protect "the most vulnerable religious minorities".
In Hamdaniyah, some 35km east of the city of Mosul, a third demonstration demanded the killers to be brought to justice.
"The blood of the innocents calls out to you: enough violence, enough terrorism," one of their banners read.
Baghdad said on Thursday it would set up an inquiry and boost security in Mosul, following the murder of an Assyrian Christian and his two sons on Tuesday.
Between 15,000 and 20,000 Christians live in Mosul, where the shrinking minority has accused authorities of inaction, noting that no murderer has been arrested or brought to justice.