HARARE, Zimbabwe  — In the first round of negotiations over how President Robert Mugabe will leave power, the Zimbabwean leader met Thursday with the army commander who put him under house arrest and mediators, including South African Cabinet ministers and a Catholic priest.

Meanwhile, an emergency summit of heads of state of regional countries was called by the 16-nation Southern African Development Community and is expected to formalize the terms of Mugabe’s exit.

The summit puts regional leaders who have worked with the 93-year-old Mugabe for decades into the difficult position of either supporting what appears to be a coup or keeping the unpopular president, the world’s oldest head of state, in office.