Muhammadu Buhari has been sworn in as Nigeria's president, promising to bring "increased prosperity" to Africa's most populous country.

He is the first opposition figure to win a presidential election in Nigeria since independence in 1960.

"I belong to everybody and I belong to nobody", he told cheering crowds at the inauguration in the capital, Abuja.

He vowed to tackle "head on" the issues of corruption and the insurgency from militant Islamist group Boko Haram.

Mr Buhari, a former military ruler, has taken over from Goodluck Jonathan, who had been in office since 2010.

'Godless militants'

At the inauguration ceremony at Abuja's Eagle Square - Mr Jonathan handed over the constitution and national flags before Mr Buhari took his oath of office.

In his first speech as president, Mr Buhari reiterated his commitment to tackle Boko Haram, whom he described as "a mindless, godless group, who are as far away from Islam as one can think".

Mr Buhari also announced plans for the Nigerian military's command centre to be moved from Abuja to the strategic north-eastern city of Maiduguri, which is closer to areas where the group operates.

He said Boko Haram could not be said to be defeated without rescuing the more than 200 Chibok girls, whose capture last April sparked a global campaign to bring them back home.

"This government will do all it can to rescue them alive," he said.

Mr Buhari said the Nigerian economy was "in deep trouble", identifying "insecurity, pervasive corruption... and seemingly impossible fuel and power shortages" as key concerns.

The country's power supply crisis was "a national shame", he said, which had brought "darkness, frustration, misery, and resignation" to Nigerians.

The president rounded off his speech with a quotation from Shakespeare, before issuing a final rallying call to Nigerians: "We have an opportunity. Let us take it."

Among the guests at the ceremony were US Secretary of State John Kerry and African leaders including Zimbabwe's President Robert Mugabe.