Archbishop Emeritus Desmond Tutu has withdrawn from a leadership summit in Johannesburg because of the attendance of former British prime minister Tony Blair, his office said on Tuesday.
"The Archbishop has spent considerable time over the past few days wrestling with his conscience and taking counsel from trusted advisers with respect to his attendance at the event," it said in a statement.
"Ultimately, the Archbishop is of the view that Mr Blair's decision to support the United States' military invasion of Iraq, on the basis of unproven allegations of the existence in Iraq of weapons of mass destruction, was morally indefensible."
It said it was "inappropriate" for Tutu to share a platform with Blair.
"The Archbishop greatly regrets inconveniencing and disappointing the organisers and participants of the Discovery Invest Leadership Summit."
Blair is one of the speakers at the summit at the Sandton Convention Centre on Thursday. Political activist Garry Kasparov was also expected to speak.
Muslim political party Al Jama-ah said on Monday it planned to hold a protest against Blair outside the convention centre during his address.
"The demonstration is being held to support a warrant of arrest to charge him for crimes against humanity relating to the invasion of Iraq which led to the killings of millions of Iraqis," said Al Jama-ah leader Ganief Hendricks.
Hendricks said the City of Johannesburg was allowing the party to hold its demonstration.
"It is hoped that one or more demonstrators will be able to make a citizen's arrest on the day and put Tony Blair in jail and extradite him to the Hague for trial."
The website arrestblair.org has offered monetary awards to people who try to make a citizen's arrest of Blair for alleged crimes against humanity.
Attempts to arrest him have been made in China, outside the Chilcot Inquiry into the Iraq war, and the European Parliament and in Dublin, Ireland.