Rodrigo Duterte sworn in as the President of Philippines
President Rodrigo Duterte takes his oath as his daughter Veronica holds the bible during his inauguration as President of the Philippines at the Malacanang Palace in Manila, Philippines Reuters

Rodrigo Duterte was sworn in as the 16th president of the Philippines on Thursday, following a massive election victory in May.

Duterte took the oath of office before a Supreme Court justice, pledging to  "preserve and protect" the constitution. He said in his inaugural speech that "true change is the mandate of my government." 

"I was elected as president to serve the entire country. I serve everyone. But not only one," the controversial leader said. 

During his election campaign, the 71-year-old former mayor of Davao City had threatened to shoot criminals and kill drug dealers. On Thursday he again promised a "relentless" and "sustained" fight against corruption, criminality and illegal drugs.

But he also said that he knows "the limits of the power and authority of the president" and he is aware of "what is legal and what is not".

"I know that there are those who do not approve of my methods of fighting criminality. They say that my methods are unorthodox and verge on the illegal," he added in his speech.

"My adherence to due process and the rule of law is uncompromising. You mind your work and I will mind mine."

As president, he also promised to honour international treaties and respect peace deals agreed with rebels at home.

Duterte served as mayor of Davao for about 22 years. During his term he built reputation for being straightforward and for supporting the extrajudicial killing of suspected criminals. Davao, which was once known as the Philippines' "murder capital" experienced a lot of changes during his rule.

He has never lost an election in three decades in politics. Duterte is also the first city mayor who has been elected as the president, without having a national position earlier.