Emmerson Mnangagwa took the oath as Zimbabwe's president in front of a stadium crowd after a divisive election. Picture: Philimon Bulawayo/Reuters

Harare - Emmerson Mnangagwa took the oath as Zimbabwe's president in front of a stadium crowd on Sunday after a divisive election, as US observers of that vote questioned the country's democratic credentials.

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

Thousands of people, some bussed in from outside the capital, and foreign leaders gathered at Harare's national stadium for the swearing-in of Mnangagwa, who just secured the 50 percent of votes he needed to avoid a runoff against Chamisa.

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

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

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 an army crackdown against opposition supporters, undermining promises that Mnangagwa made during campaigning to break with the corruption and mismanagement that become endemic under Mugabe.

The crackdown, which left six people dead on August 1, 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 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.