Rupiah Banda, Zambia's acting president since the death of Levy Mwanawasa in August, has been sworn in as the new president after narrowly beating his rival in a disputed election.
Banda, a pro-business centrist, won 40.1 per cent of the vote against 38.1 per cent for Michael Sata, the leader of the oppsition Patrotic Front.
Sata has, meanwhile, rejected the results and accused the ruling Movement for Multiparty Democracy (MMD) of rigging. He is seeking a recount of the votes.
"I declare today that I will maintain and uphold the constitution and laws of Zambia, so help me God," Banda said on Sunday as he took the oath of office in Lusaka, the capital.
The election commission dismissed charges of rigging and African observers have declared the vote free and fair.
Given Lubinda, a spokesman for the Patriotic Front, said: "Our stand is still that we do not recognise the election of Rupiah Banda as something reflecting the will of the people of Zambia.
"We are going to ask the court (on Monday) to grant an order to scrutinise and recount the votes."
Accusations of electoral fraud have led to fears of unrest.
Banda overcame a 30-point deficit during two days of vote counting, after trailing in initial counting.
He enjoyed rural support while Sata did well in urban areas.
"I have evidence that results are being inflated ... They cheated me in 2006 and they want to do the same," Sata said at a conference hall in Lusaka.
Officials in Africa's largest copper producer have said that the army is on standby in case tensions escalate.
Ronnie Shikapwasha, the home affairs minister, said: "Defence forces are on alert in border areas and other places to quell firmly any violence."