Delegates from Ghana's ruling party are choosing their candidate for the coming presidential election – the first time a sitting president has had to compete in a primary for his party's nomination.

Around 3,000 delegates were expected to turn out Saturday for the National Democratic Congress party primary being held in the city of Sunyani in southwestern Ghana.

The ballot pits President John Atta Mills against Nana Konadu Agyeman Rawlings, wife of former president Jerry Rawlings.

She announced in April she would seek the nomination, in a sign of divisions among the ruling party ahead of elections set for December 2012.

Her husband, Jerry Rawlings, seized power in Ghana in successive coups, first in 1979 and then in 1981. Jerry Rawlings then served as Ghana's elected president from 1993 to 2001.

President Mills launched his campaign for re-election in May, expressing confidence he would win the nomination.

The former opposition leader took office in 2009, succeeding John Kufuor who served the maximum of two four-year terms.