Emmerson Mnangagwa has been officially sworn in as president of Zimbabwe after winning a bitterly contested election that marked the country's first vote since Robert Mugabe was ousted from power.

Mr Mnangagwa, whose victory in the 30 July polls was challenged by the main opposition, pledged to "protect and promote the rights of Zimbabweans" in an inauguration ceremony attended by a crowd of thousands at a stadium in Harare.

"I Emmerson Dambudzo Mnangagwa swear that as president of the republic of Zimbabwe I will be faithful to Zimbabwe (and) will obey uphold and defend the constitution of Zimbabwe," he said.

Once Mr Mugabe's right-hand man, Mr Mnangagwa of the ruling ZANU-PF party won the election with 50.8% of the vote - just enough to meet the 50% threshold needed to avoid a run-off against the main opposition led by Nelson Chamisa, who scored 44%.

International observers said the polls were largely free of the violence which characterised previous votes in Zimbabwe.

Zimbabwe's top court on Friday dismissed Mr Chamisa's bid to have the results annulled because of allegations of vote rigging.

Mr Mnangagwa took the oath before Chief Justice Luke Malaba who, together with eight other Constitutional Court judges had dismissed Mr Chamisa's petition.

The election was touted as a crucial step towards shedding the pariah reputation Zimbabwe gained under Robert Mugabe, and securing international donor funding to revive a crippled economy.

Nelson Chamisa's bid to have the results annulled because of vote rigging were dismissed 

But hours before Mr Mnangagwa's inauguration, the International Republican Institute and National Democratic Institute said the country lacked a "tolerant democratic culture" in which political parties were treated equally and citizens allowed vote freely.

The election was marred by procedural lapses and followed by an army crackdown against opposition supporters, undermining promises that Mr Mnangagwa made during campaigning to break with the corruption and mismanagement that become endemic under Mr Mugabe.

The crackdown, which left six people dead on 1 August, recalled the heavy-handed security tactics that marked the 37-year rule of Mugabe, who was removed in a coup in November.

The US observers also urged "all sides to rely on peaceful expression and to avoid acts or threats of retribution against political rivals following the Constitutional Court's decision."

Washington has maintained travel and financial sanctions on senior ruling party officials, including Mr Mnangagwa, as well as some state-owned firms.

Washington's support is key if Zimbabwe is to get any funding from the International Monetary Fund.

The European Union, meanwhile, has progressively removed sanctions and they only remain in place on Mr Mugabe and his wife Grace.