Emmerson Mnangagwa urged Zimbabwe to unite behind his presidency on Sunday, as he took the oath of office following a divisive election that US observers said had called the country's democratic credentials into question.

The Constitutional Court confirmed Mnangagwa as president on Friday, dismissing a challenge by the man he beat in the July 30 ballot, Nelson Chamisa.

Thousands of Zimbabweans, some bussed in, and foreign leaders including South Africa's Cyril Ramaphosa and Paul Kagame of Rwanda gathered at Harare's national stadium for Sunday's swearing-in.

"I exhort us to commit ourselves collectively to develop our motherland... what unites us is greater than what could ever divide us," Mnangagwa told election participants in his inauguration speech.

He also reaffirmed pre-election pledges to revive Zimbabwe's crippled economy and settle outstanding debts with foreign lenders, and reiterated he would call an independent inquiry into a "regrettable and unacceptable" army crackdown following the vote in which six people died.

"Now is the time for us all to unite as a nation and grow our economy," Mnangagwa said.

He took the oath before Chief Justice Luke Malaba who, together with eight other judges, had ruled on Friday against opposition leader Chamisa's petition.

In the election, Mnangagwa just reached the threshold of 50 percent of votes that he needed to avoid a runoff.

NO LONGER A PARIAH?

The ballot was touted as a crucial step towards shedding the pariah reputation Zimbabwe gained under Mnangagwa's predecessor Robert Mugabe, and securing international donor funding.

But hours before 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 to vote freely.

The election was marred by procedural lapses and followed by the crackdown against opposition supporters, which recalled the heavy-handed security tactics that marked Mugabe's 37-year rule.

Those events tarnished promises that Mnangagwa made during campaigning to break with the corruption and mismanagement that become endemic under Mugabe, who was removed in a coup in November.

Mugabe, who has accused Mnangagwa - his former head of intelligence and defence minister - of betrayal, did not attend Sunday's ceremony, though the 94-year-old former leader's daughter Bona did. In keeping with an earlier promise, Chamisa also stayed away.

Citing the Constitutional Court ruling, the US observers also urged all parties "to rely on peaceful expression and to avoid acts or threats of retribution against political rivals."

Washington has maintained travel and financial sanctions on senior ruling party officials, including 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 Mugabe and his wife Grace.