Joao Lourenco has been sworn in as president of Angola, becoming the first new leader in the oil-rich southern African nation in almost four decades.

Lourenco, 63, took over on Tuesday from Jose Eduardo dos Santos, who ruled the former Portuguese colony for 38 years.

The People's Movement for the Liberation of Angola (MPLA) has governed since Angola's hard-fought independence from Portugal in 1975, with Dos Santos taking power in 1979.

The party won 61 percent of the vote in August, a sharp drop in support from the previous election in 2012 as the country remains in the grip of an economic crisis prompted by the fall in oil prices.

"Employment, poverty alleviation, promotion of opportunities and business policies will be implemented," Lourenco said, adding he would govern for "all Angolans".

Opposition parties boycotted the inauguration after denouncing "irregularities" in the election, saying the ballot was marred by unfair media coverage and suppression of opposition voters.

UNITA, the main opposition party, this month dropped its threat to refuse to attend parliament over its complaints, while the Consitutional Court dismissed its case alleging election flaws.

Lourenco will be Angola's first head of state to be constitutionally limited to serving two five-year terms.