People in Ghana take to the streets to call for change
Ghana poll monitors are probing claims of fraud as partial unofficial results suggest an extremely close race in Sunday's presidential run-off.
Local media report the opposition's John Atta Mills has a slender lead over the governing party's Nana Akufo-Addo.
The two parties are predicting victory despite claims by both of intimidation and voting irregularities.
Mr Akufo-Addo was narrowly ahead of his rival in the first round on 7 December but not by enough to avoid the run-off.
According to partial unofficial results, the opposition's Mr Atta Mills has 4.35 million votes against 4.24 million for Mr Akufo-Addo, from 222 out of a total 230 constituencies, reports the website of privately-owned Joy FM radio.
The elections are the fifth since the country's return to democracy in 1992
Outside the headquarters of the opposition National Democratic Congress (NDC) in the capital Accra people have been dancing in the streets in anticipation of what they believe is a victory.
However John Atta Mills has urged his supporters to wait for the official electoral commission result before celebrating.
Across town people at the headquarters of the governing New Patriotic Party (NPP) were still confident that Nana Akufo Addo would become the next president.
"We believe in figures. Now we know, we are sure that we have won," one supporter told the BBC.
The final official outcome is expected on Tuesday.
Both men hope to succeed President John Kufuor, who has served two terms.
The stakes are high as Ghana has just found oil.
The BBC's Will Ross in Accra says the possibility of the result being disputed is high, as there were some irregularities on polling day and both parties are claiming foul play.
RIVALS AT A GLANCE
JOHN ATTA MILLS (left)
Party: National Democratic Congress
Executive posts: Vice-president 1997-2000
Hobbies: Hockey, swimming
Family: Married with one child
NANA AKUFO-ADDO (right)
Party: New Patriotic Party
Executive posts: Attorney general 2001-03; foreign affairs minister 2003-07
The ruling NPP said its polling agents were intimidated in the opposition stronghold of Volta Region.
NPP campaign chairman Jake Obetsebi Lamptey said:
"In these circumstances where our agents have been unable to monitor the voting process they will not be able to endorse the declared results."
The opposition has similar complaints in the governing party stronghold of the Ashanti region.
NDC officials have told the BBC that half of its polling agents there were harassed, had to flee and were not able to endorse the results.
But our correspondent says some may view this as an excuse by both sides in case the final result does not go their way and there is a danger this vote could be heading for acrimony.
The Coalition of Domestic Election Observers said it would investigate dozens of cases of irregularities reported at polling stations in various parts of the country.
It said it was "concerned with the rising number of incidents in the run-off compared to the December 7 election," reported AFP news agency.
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.
The US envoy to Africa, Jendayi Frazer, urged both parties not to spread rumours.
"Some of the statements by the political parties' officials have been irresponsible and there are a number of rumours that have been going on that are being repeated by some of the radio stations that are associated with the parties," she said.
In Accra and across the West African state during Sunday's vote, queues formed outside schools and other public buildings where polling stations were operating, guarded by armed soldiers and police.
Mr Akufo-Addo won the first round with 49% of the vote.
President Kufuor, who has to stand down having served two consecutive terms, urged Ghanaians to "keep cool" on the eve of the poll.
Ghana, traditionally an exporter of cocoa and gold, is preparing to start producing oil from late 2010.
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