Sierra Leone's new president has pledged to fight corruption and help reduce abject poverty in front of the crowds at his official inauguration.
Mr Koroma was elected on an anti-corruption platform
Ernest Bai Koroma said there should be progress within three years and a change to people's mentality.
"The police must stop taking tips from the driver and the teachers must stop taking handouts from kids and parents," he told the BBC before the ceremony.
A BBC reporter says the handover is the most orderly Sierra Leone has seen.
Sierra Leone, one of the poorest countries in the world, is very slowly recovering from a decade of brutal war that ended in 2001.
The inauguration ceremony in the national football stadium was watched by five African heads of state and tens of thousands of Mr Koroma's supporters, dressed in red and white.
The BBC's world affairs correspondent Mark Doyle says it was so hot in the stadium that some of the soldiers fainted.
Other people also fainted but the Sierra Leonean Red Cross were doing their best to look after them, he says.
President Koroma's anti-corruption policy has been welcomed by foreign diplomats.
"I have made the position very clear that we have to take on corruption and I have advised everybody, including members of my family that it is going to be a serious business this time," Mr Koroma said.
He made the remarks after a report he commissioned showed widespread graft under the previous administration.
But our correspondent says sceptical Sierra Leoneans say the president will be judged on his actions, not his words.
They say they will be watching to see if any high level prosecutions take place.
Laws and regulations will have to be tightened up but a whole culture and mentality will also have to change.
Our correspondent says the inauguration has stretched the logistical resources of Sierra Leone to the limit.
It did not have helicopters, for example, to bring the heads of state across the wide river estuary that separates the international airport from the capital city - so it borrowed some from the United Nations.
It also did not have access to enough limousines to ferry the visiting VIPs around, so it was given some by friends, including some cars reportedly from nearby Senegal.