Earlier, Mrs. Banda declared 10 days of mourning, and ambassadors, the army chief, government officials and other prominent figures have been streaming to her home in Lilongwe.

She was once an ally of Mr. Mutharika, but the two fell out over efforts to groom the president’s brother, Peter, as a successor. Mrs. Banda then formed her own political party. On Friday, the information minister, Patricia Kaliati, hinted that the president’s party, the Democratic Progressive Party, did not see Mrs. Banda as a legitimate successor because she had been expelled from the governing party.

But with the military, the judiciary and an increasing number of cabinet members backing Mrs. Banda, any effort to replace her would face long odds of success, analysts said.

Mr. Mutharika, 78, had a heart attack on Thursday, but the government refused to confirm his death until Saturday. A former World Bank economist, Mr. Mutharika had been popular for his stewardship of Malawi’s economy, presiding over a period of extraordinary growth and increased farm yields thanks to popular fertilizer subsidies. He was first elected in 2004 and then returned to power in 2009.