Tensions are high in Ghana as official results from the knife-edge run-off presidential poll are to be released.

Opposition candidate John Atta Mills claims he has won, but the governing NPP party insisted the result remained too close to call.

Police earlier fired warning shots as large crowds converged on the election headquarters demanding the results.

Observers, who have given the poll a preliminary clean bill of health, urged both candidates to accept the results.

Monitors hope Ghana's poll can help salvage the tarnished image of constitutional democracy in Africa, after flawed elections in Kenya and Zimbabwe and military coups in Mauritania in August and in Guinea last week.

Besieged

The electoral commission headquarters in Accra was besieged on Tuesday by hundreds of opposition National Democratic Congress (NDC) supporters demanding their candidate be declared the winner.

Ahead of a news conference scheduled for 1200GMT to release the official results, armed police and soldiers backed by water cannon trucks and an armoured personnel carrier kept the protesters behind barricades.

Governing New Patriotic Party chairman Peter Mac Manu has said there was widespread intimidation of its election agents in the Volta region and results from these areas would be challenged.

NPP candidate Nana Akufo-Addo gained the most votes in the first round earlier this month but did not pass the 50% threshold needed for outright victory.

The BBC's Will Ross in Accra says that with allegations of vote-rigging on both sides in an extremely tight race, there are fears violence could break out.

The stakes have been raised in these elections because of the discovery of oil, which is expected to start generating revenue in 2010.

Outgoing President John Kufuor has appealed for calm and for people to wait for the final results.

Hundreds of opposition NDC supporters have taken to the streets in the party colours of red, white, green and black to celebrate reports of Mr Atta Mills' victory.

He told his supporters: "The figures show clearly that I have won the election. I am only waiting for the electoral commissioner to declare me winner."

The Electoral Commission on Monday declared results from 200 of the 230 constituencies, which showed Mr Atta Mills leading with 52% of the vote, against 48% for Mr Akufo-Addo.

Unofficial results from privately owned radio station Joy FM say Mr Atta Mills has an unassailable lead with just one constituency still to declare its results.

But state broadcaster Ghana Broadcasting Corporation (GBC) says Mr Akufo-Addo now has a slight lead, also based on results from 229 constituencies.

GBC results had earlier put Mr Atta Mills narrowly ahead.

Mr Mac Manu said unofficial predictions that Mr Atta Mills had won were "highly speculative and premature".

Ghana is seen as one of Africa's most democratic countries and is hoping to restore the continent's faith in elections after flawed polls and violence in Kenya and Zimbabwe this year.

Some 12.5 million people were eligible to vote in the election - the fifth since the country's return to democracy in 1992.

President Kufuor is standing down having served two consecutive terms.

He defeated Mr Atta Mills in the two previous elections.

Mr Atta Mills served as vice-president under former leader Jerry Rawlings.