"You can go to hell. Mr. Obama, you can go to hell," he told reporters at one news conference.
"Some industrialized nations such as the US and members of the EU, are criticizing my crackdown on drugs. It's silly," he has also said.
Davao is the city where he served as mayor for about 2 decades. Bonifacio Tan, president of the city's Chamber of Commerce and Industry, says the local economy was at rock bottom before Duterte became mayor. Abductions of wealthy people were rampant, and making money was risky because it would often draw the attention of kidnappers.
"Sometimes killing, so no expansion, they were afraid," Tan says.
Duterte strived to fully restore public security, and he began by overhauling the city police, expelling one corrupt officer after another. Local media say suspected criminals were killed by vigilantes thought to be connected with Duterte.
As security improved, the city saw an increase in investments. Over the last 2 decades, there has been a 14-fold rise in the number of businesses.
"The government just can make a peace and order and the infrastructure so that businessman can grow," Tan says.
Some point to his time working as a public prosecutor as the beginning of his aspirations to the presidency. Antonina Oshita Escovilla was a colleague of Duterte's at that time.
"He wanted to become a prosecutor to eliminate crimes," Escovilla says.
But Escovilla also points out that Duterte was frustrated by the limitations of the legal system.
"Even during the early days, there is already corruption, even during our times. But when you become mayor, you can change an entire city. Now if you become president, you can make some changes as a whole," Escovilla says.
One of Duterte's close aides says his scornful comments about the US come from an experience in his days as mayor. Melchor Quitain is a senior official at the Presidential Office. He has assisted the President since he was mayor of Davao, and he says that when a bombing occurred in Davao, the US authorities took the suspect to their country.
"Maybe he thought there is a chance for us to correct historical error," Quitain says.
Behind Duterte's controversial statements are his experiences as mayor of Davao. Now all eyes are on how he will lead his country from this point on.