MANILA - Populist firebrand Rodrigo Duterte took office at noon on Thursday (June 30) as the Philippines' 16th president, ushering in a government bent on waging a "relentless" and "sustained" war on crime, while spreading prosperity to "those who have little".

"The fight will be relentless, and it will be sustained," the 71-year-old Mr Duterte told some 600 guests in a 16-minute speech shortly after being sworn in at the Malacanang Palace to succeed Mr Benigno Aquino.

True to character, the former Davao Mayor defied convention by taking his oath before a small audience inside the presidential palace, instead of at a big public rally like previous Filipino leaders.

"The ride will be rough, but come and join me just the same. Together, shoulder-to-shoulder, let us take the first wobbly steps in this quest," he said

Departing from his brash, usually profanities-laced, mien, a muted Mr Duterte again hammered on the central themes of his six-year rule over the country of 100 million people: ending crime and ridding the bureaucracy of graft.

He has been credited to improving law and order in formerly crime-ridden Davao during his 22 years as Mayor. But right groups accuse him of links to vigilante death squads which are believed to have killed more than 1,000 people.

Mr Duterte has promised to end crime in six months by any means, including extrajudicial killings. He has threatened to send the bodies of tens of thousands of criminals to the bottom of the sea and vowed to reinstate the death penalty.

Security forces and unnamed vigilante groups had begun shooting down suspected drug kingpins and their dealers since he swept to victory on May 9, with over 50 men linked to the narcotics trade having been killed.

In his inauguration speech on Thursday, Mr Duterte asked lawmakers and human rights campaigners "to allow us a level of governance consistent with our mandate". "You mind your work, and I will mind mine," he said.

He said "corruption, both in the high and low echelons of government, criminality in the streets and rampant sale of illegal drugs" are "the problems that bedevil our country today which needs to be addressed with urgency".

"They say my methods are unorthodox and verge on the illegal. In response, let me say this… I have seen how illegal drugs destroy individuals and ruin family relationships. I have seen how criminality, by means all foul, snatch from the innocent and the unsuspecting the years and years of accumulated savings, years of toil, and then suddenly they are back to where they started," he thundered.

"Look at it from that perspective, and tell me that I am wrong."

On dealing with poverty, Mr Duterte channelled former US president Franklin Roosevelt, saying: "The test of our progress is not whether we add more to the abundance of those who have mch. It is whether we provide for those who have little."

Mr Duterte has eschewed discussing economic policies at length, but said he believed growth would follow when investors are assured they can do business in a crime-free and level playing field.

He indicated on Thursday he would also make it easier for businesses by promoting transparency in awarding government contracts:

"I order all department secretaries and heads of agencies to refrain from changing and bending the rules government contracts, transactions and projects already approved and awaiting implementation. Changing the rules when the game is on-going is wrong," he said.

Among those attending his inauguration were former presidents Fidel Ramos and Joseph Estrada, who sat next to Mr Duterte.

Also in the hall were communist nominees to Mr Duterte's Cabinet, and chief executives of companies often at odds with leftist groups over their labour policies.

In a sign of the seismic political shift that has come with Mr Duterte's rise, two children of the late dictator Ferdinand Marcos Jr were also in attendance.

Mr Aquino's father and namesake was killed in 1983 by assassins believed to be henchmen of Marcos' cronies.

That precipitated an uprising that culminated in a "People Power" revolt that forced the Marcoses to flee Malacanang, and installed Mr Aquino's mother, democracy icon Corazon Aquino, as president.

In a symbolic rite shortly before he took his oath, Mr Duterte ascended a wide staircase where he was greeted by Mr Aquino.

After a brief meeting, Mr Aquino descended the same staircase, received departure honours, and was driven out of Malacanang, no longer a president, but an ordinary citizen.