Mr Mwanawasa chairs the Southern African Development Community
Zambian President Levy Mwanawasa is being treated for a mild stroke in the Egyptian resort of Sharm el-Sheikh, Zambia's vice-president has said.
Mr Mwanawasa's condition was described as stable.
The president, 59, was rushed to hospital on Sunday with sharp chest pains ahead of an African Union summit.
The summit was expected to address Zimbabwe's disputed election. Mr Mwanawasa has taken a tough line against Robert Mugabe's regime.
"I wish to inform the nation that the president had suffered a stroke," Zambian Vice-President Rupiah Banda said in a statement.
"However, upon being attended to by doctors his condition has been described as stable."
Mr Mwanawasa, the current chairman of the Southern African Development Community (Sadc), said he sympathised with Zimbabwean opposition leader Morgan Tsvangirai when he withdrew from the presidential election because of political violence against his supporters.
"Elections held in such an environment will... bring embarrassment to the Sadc region and the entire continent of Africa," he said then.
He has compared the economic situation in Zimbabwe to "a sinking Titanic".
The Zambian president had been due to sit next to Mr Mugabe at the AU summit, where seating is arranged alphabetically.
Egypt's state news agency Mena reported that the Egyptian Foreign Minister Ahmed Aboul Gheit was monitoring Mr Mwanawasa's treatment in Sharm el-Sheikh, and that President Hosni Mubarak had visited him in hospital.