( CNN ) -- Ghanaian officials swore in a new president Tuesday , hours after the death of the West African nation 's leader . John Evans Atta Mills died suddenly at a military hospital Tuesday afternoon a few hours after becoming ill , Chief of Staff John Henry Martey Newman said in a statement . He was 68 . In a ceremony broadcast on state television , John Dramani Mahama -- formerly Ghana 's vice president -- became the country 's new president and ordered that flags be flown at half-staff for a week . `` This is the saddest day in our nation 's history . Tears have engulfed our nation and we 're deeply saddened and distraught . I never imagined that one day I will address our nation in such difficult circumstances , '' Mahama said . `` I 'm personally devastated . I 've lost a father , I 've lost a friend , I 've lost a mentor and a senior comrade . '' Opinion : Mills ' election victory was a 'victory for democracy in Africa ' Officials did not specify what caused Mills ' death . The president had denied rumors about his health for months . `` Does my continued living pose a threat to some people ? '' he told reporters at a January event , according to the state-run Ghana News Agency . On June 25 , he returned from a medical checkup in the United States , the news agency reported . Mills was a former law professor and a tax expert . He was Ghana 's vice president from 1997 to 2000 . Before his political career , he taught at the University of Ghana and also was a visiting lecturer at Temple University in Pennsylvania and Leiden University in the Netherlands . Mills ran for president unsuccessfully in 2000 and 2004 before narrowly winning a runoff in 2009 . Mills ' death came several days after he celebrated his 68th birthday . He had said he would run for re-election in December . In a statement read on his behalf by Ghana 's information minister Monday , Mills said his government had focused on developing the nation 's economy , according to the Ghana News Agency . `` We made a promise to expand our country 's infrastructural base in support of our economy take-off , '' the statement said . `` We also decided to invest in the people of our country so as to make them competitive both locally and abroad . '' Mahama said Tuesday that Mills was a `` prince of peace '' who `` brought a distinctive insight into Ghanaian politics . '' The opposition New Patriotic Party expressed condolences in a statement Tuesday . `` We join the nation in mourning this sad loss to Ghana , '' the statement said . U.S. President Barack Obama met with Mills when he visited Ghana in July 2009 . Obama praised the country as a model for democracy and stability when Mills visited Washington this year . `` Ghana has become a wonderful success story economically on the continent , '' Obama said . `` In part because of the initiatives of President Mills , you 've seen high growth rates over the last several years . Food productivity and food security is up . There 's been strong foreign investment . '' In a statement Tuesday , British Prime Minister David Cameron described Mills as `` a tireless defender of democracy in West Africa and across the continent . '' Part of a former British colony , Ghana was among the first African countries to gain independence , in 1957 . It endured a series of coups before a military dictator , Jerry Rawlings , took power in 1981 . Rawlings led Ghana through a transition to democracy about 10 years later . Mahama , 53 , is a former member of Ghana 's parliament who has served as director of communication for the National Democratic Congress party . On Tuesday , the new president called on Ghanaians to respect Mills ' legacy . `` Our finest tribute to him at this moment is to maintain the unity and stability of our nation , '' Mahama said . Obama hailed Ghana as 'model for democracy ' Share your memories of Mills with CNN iReport . Journalist Israel Laryea contributed to this report .
NEW : ` ` This is the saddest day in our nation 's history , '' the new president says . John Evans # ##way died a few hours after becoming ill , his chief of staff said . The president was a former law professor and tax expert . The president died at a military hospital , the chief says .