His African National Congress (ANC) party lost its controlling majority in last month's election after ruling for 30 years since the end of apartheid.

President Ramaphosa's motorcade arrived at the ceremony in Pretoria on Wednesday, with a guard of honour on horseback, watched by African heads of state and dignitaries.

Supporters waved flags at the proceedings and a formation of aircraft conducted a flypast overhead as he took the oath of office.

"In our brilliant diversity, we gather to affirm our solemn conviction that this country belongs to all who live in it," the president told those gathered.

"Understanding that no party can govern alone and make laws alone, these parties have agreed to work in partnership, to employ their talents for the good of the country and the progress of its people."

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President Ramaphosa will head what he calls a government of national unity with five other parties, including the ANC's largest rival and virulent critic, the pro-business Democratic Alliance (DA).

While investors have welcomed the inclusion of the DA, analysts say sharp ideological divisions between the parties could make the government unstable.

Just before the election, President Ramaphosa signed into law a National Health Insurance bill that the DA says could collapse a creaking health system.

It was unclear what would happen to that law under the new government. The DA advocates scrapping the ANC's flagship black economic empowerment programme, saying it hasn't worked - a highly contentious topic in a nation grappling with huge inequalities, some inherited from apartheid.