People across Ghana are mourning the sudden death of president John Atta Mills, as the country's vice president was sworn-in to serve out the remainder of his term.

Vice President John Dramani Mahama took the oath of office Tuesday evening hours after officials announced the death of Mr. Mills.

In an address Tuesday night, Mr. Mahama described the death as “the saddest day in (the) nation's history.” He added that Ghanaians are “united in grief” for their departed president.

No public announcement has been made about the cause of Mr. Mills death, or when a funeral will be held.

The smooth transition of power underscores Ghana's role as one of the continent's most stable democracies. It is also a large-scale oil producer, has vast gold mines and is the world's second largest cocoa exporter.

The 68-year-old Mills was elected in a 2008 vote and was expected to run in December's presidential election.

The main opposition New Patriotic Party has suspended its campaign to honor Mr. Mills and to help unite the nation.

Chairman Jake Obetsebi-Lamptey praised Ghana's handling of the crisis.

“We are showing a maturity that must encourage all Ghanaians.”

Political analyst Emmanuel Akwetey, executive director of the Institute for Democratic Governance, told VOA that he expects the ruling National Democratic Congress Party to name Mr. Mahama as its candidate. He noted that Mr. Mahama campaigned with the late president and that the time remaining before the vote is short.