Malawi's President Mutharika treated for cardiac arrest
Malawi's President Bingu wa Mutharika has been flown to South Africa for medical treatment after suffering a heart attack, state media report.
Mr Mutharika, 78, was admitted to the intensive care unit of a hospital in the capital, Lilongwe, after falling ill on Thursday morning.
The president has been under mounting pressure to resign, amid accusations of nepotism and economic mismanagement.
Reports say police have deployed in force across the capital.
A senior official at Kamuzu Central Hospital told AFP news agency that the president had been unconscious since 09:00 local time (07:00 GMT) on Thursday.
The official, who did not want to be named, described his condition as critical.
His wife and his brother, Foreign Minister Peter Mutharika, are among those to have visited him, sources say.
A hospital source quoted by the Reuters news agency said there had been "panic" when the president was brought in.
"His head was covered to hide his face as he was wheeled in on a wheelchair," another medical source told the agency.
For most of the day, state media made no mention of the news and officials denied the reports.
But on Thursday evening, Vice-President Joyce Banda confirmed that the president was unwell.
"When I first heard the state president had been taken ill, I quickly wished him quick recovery. I wish to ask all Malawians to pray for the quick recovery of our state president," said Mrs Banda.
Correspondents say there is concern that his death could spark a political crisis in the impoverished African nation.
Discontent with his leadership have recently sparked violent protests against his rule.
The president has also been embroiled in a bitter power struggle within his Democratic Progressive Party (DPP).
Under the terms of Malawi's constitution, Ms Banda takes over if the head of state is either incapacitated or dies.
But Ms Banda and Mr Mutharika fell out after a row over the succession in 2010, and she was expelled from the ruling party.
The BBC's Raphael Tenthani in Blantyre says that if such a power transfer comes to pass, it would be a huge shake-up of Malawi's political establishment.
Bingu wa Mutharika came to power in a 2004 election. Soon afterwards, he dramatically abandoned his United Democratic Front (UDF) party to form the DPP, after accusing leading UDF members of opposing his campaign against corruption.
Since being re-elected with a large majority in 2009, critics allege he has demonstrated an increasingly authoritarian streak.
The criticism has led to a souring in relations with major foreign aid donors, especially the United Kingdom.
Last year, Mr Mutharika expelled the UK High Commissioner, Fergus Cochrane-Dyet, after a leaked embassy cable quoted the diplomat as saying that the president could not tolerate criticism.
The Malawian leader said he could not accept "insults" just because the UK was his country's largest aid donor.
In response, the UK expelled the Malawian envoy to London and cut direct aid.
Malawi is one of the poorest countries in the world, with an estimated 75% of the population living on less than $1 (60p) a day.
The country has suffered shortages of fuel and foreign currency since the UK and other donors cancelled aid.