Emmerson Mnangagwa has been sworn in as Zimbabwe’s president a day after his main challenger reaffirmed his rejection of the election result.
Earlier on Sunday US election observers said the country lacked a “tolerant democratic culture”.
On Friday the Constitutional Court rejected the opposition’s claims of fraud, saying there was no evidence.
July’s poll was the first since long-time leader Robert Mugabe was ousted from power last year.
Two days after the vote, at least six people were killed in clashes between the army and supporters of the opposition MDC Alliance who alleged that party leader Nelson Chamisa had been robbed of victory.
Hours before Mr Mnangagwa’s inauguration, the US International Republican Institute and National Democratic Institute said Zimbabwe’s system did not allow political parties to be treated equally and citizens to vote freely.
They urged all sides to avoid “acts or threats of retribution against political rivals following the Constitutional Court’s decision”.
Mr Mnangagwa, from the Zanu-PF party of Mr Mugabe, sworn in at an event attended by thousands of people.
He took the oath before Chief Justice Luke Malaba, who was among the judges who dismissed the MDC leader Nelson Chamisa’s claims of vote-rigging.
But after the ruling Mr Chamisa insisted that he had a “legitimate claim” to lead the country and vowed to lead peaceful protests.
“The court’s decision is not the people’s decision,” Mr Chamisa said.
Mr Chamisa says he rejects the court decision upholding the result
The US has maintained some sanctions on senior Zanu-PF officials including Mr Mnangagwa. The EU has removed most of its sanctions.
EU election observers said after the Constitutional Court ruling that all parties should accept the verdict but also suggesting that President Mnangagwa had benefited from an “un-level playing field”.