Shortly after he was sworn-in as president, John Mahama paid a glowing tribute to his former boss whom he described as a friend, father, senior comrade and mentor. He however said all flags in the country will run at half mast for the next one week as the country mourns its departed president.

Mr Atta Mills had ruled the West African country since 2009.

"It is with a heavy heart... that we announce the sudden and untimely death of the president of the Republic of Ghana," the president's office statement said.

While Mr Atta Mills' illness had always been a subject of great debate, it was never officially confirmed. He had always insisted he was well, and planned to seek re-election in December's poll.

John Evans Atta Mills served as vice-president to Jerry Rawlings between 1997 and January 2001.

He came to power after narrowly winning against a candidate from the then governing New Patriotic Party, Nana Akufo-Addo, in polls in December 2008.
His predecessor, John Kufuor, stepped down after having served the maximum permitted two four-year terms.

Under Mr Atta Mills' leadership, Ghana joined the ranks of the world's large-scale oil producers.

This is the first time that a president has died while in office in Ghana. In a country hailed as a solid democracy, John Atta Mills' sudden death should not spark a political crisis but will certainly test the country's democratic institutions.

As Vice-President John Dramani Mahama steps in as interim head of state, the question people are asking in the Ghanaian capital Accra is: "Who is going to run for president with the ruling party in December?"

Mr Atta Mills had just been nominated by the National Democratic Congress to stand for a second term. But his nomination had illustrated a split within the ruling party with Nana Konadu Agyemang-Rawlings, the wife of former ruler Jerry Rawlings, who leads a faction critical of Atta Mills' management.

Mr Atta Mills' health has always been a subject of great debate in Ghana, even before he took office in 2009. But his illness was never officially confirmed and Mr Atta Mills himself insisted he was doing well.