By Kwasi Kpodo and Richard Valdmanis
ACCRA (Reuters) - Ghana's main opposition party said on Tuesday it would challenge in court the result of last week's election, which gave victory to incumbent president John Dramani Mahama with 50.7 percent of the vote.
New Patriotic Party leader Nana Akufo-Addo, who lost to Mahama in the poll, said electoral workers had tampered with the result of the December 7 vote - which was widely seen as a test for democracy in one of Africa's most stable nations.
"We are taking our matter to the place where the constitution says we should take our matter," Akufo-Addo told thousands of supporters in the same field in the capital Accra where Mahama held his victory party the previous day.
"We are going to put ourselves in the hands of the judges of the Supreme Court and they will decide the fate of this election."
The United States and regional leaders have issued statements congratulating Mahama on his victory and praising the conduct of the election, which observers said was free and fair despite some logistical glitches.
President Mahama, who took over from former leader John Atta Mills after his death in July, said he was not worried by the NPP's legal challenge and was confident supporters of the rival political camps would remain calm.
"Ghana's democracy is solid, and there are constitutional institutions to deal with situations like this," he told Reuters in an interview at his residence in Accra.
Mahama, during his vistory speech on Monday, urged his defeated rivals to drop any grievances and join him "as partners" in making Ghana better.
The gold, oil and cocoa-producing nation has managed more than 30 years of democratic transfers of power, making it an anomaly in West Africa's so-called "coup belt".
But a contested election in 2008, in which Akufo-Addo lost by less than 1 percent, pushed the country to the brink of chaos, with disputes over results driving hundreds of people into the streets with clubs and machetes.
(Reporting by Kwasi Kpodo; Writing by Bate Felix; Editing by Joe Bavier and Andrew Roche)