Duterte declared martial law in the southern region of Mindanao, home to about 20 million people, on Tuesday night after gunmen who had declared allegiance to the Islamic State group rampaged through a southern city.
At least three security force personnel died on Tuesday as they battled the militants in Marawi, a mainly Muslim city of about 200,000 people, with authorities reporting that gunmen burned a Catholic Church and other buildings.
"Our fellow Filipinos, do not be too scared," Duterte said from Moscow, where he had just begun an official visit that he abruptly ended to fly home and deal with the crisis.
Duterte vowed to be ruthless in quelling the terrorism threat in Mindanao, drawing parallels with martial law imposed by dictator Ferdinand Marcos during his two-decade rule that ended with a "People Power" revolution in 1986.
"It could not be any different from what President Marcos did," Duterte said, as he reminded Filipinos of his election campaign pledge last year to be "harsh" in dealing with terrorism.
"What I told everyone, do not force my hand into it. I have to do it to preserve the Republic of the Philippines, the Filipino people," he said.
Duterte also said that martial law in Mindanao could last up to a year.
Critics of Duterte, who has waged a controversial war on drugs that has claimed thousands of lives, have feared that he may use various security threats as an excuse to impose a form of authoritarian rule.
Opposition Senator Francis Pangilinan, president of the Liberal Party, expressed concern on Wednesday that martial law could lead to military abuses, citing extrajudicial killings under Marcos.
"Our painful experience with the imposition of martial law under the Marcos dictatorship should serve as a reminder that we must, as citizens, stay vigilant," Pangilinan said in a statement.
Deadly clashes
The fighting in Marawi erupted on Tuesday afternoon after security forces raided a house where they believed Isnilon Hapilon, a leader of the infamous Abu Sayyaf kidnap gang and Philippine head of IS, was hiding.
The United States regards Hapilon as one of the world's most dangerous terrorists, offering a bounty of $5 million for his capture.
More than 100 gunmen responded to the raid by burning buildings and conducting other diversionary tactics, according to Defence Secretary Delfin Lorenzana.
Photos posted on social media by residents showed the gunmen walking through the streets of Marawi and placing black flags that looked similar to those used by IS.
Lorenzana said on Tuesday night that many gunmen were hiding in buildings as snipers, making it difficult for security forces to combat them.
There were no reports of fighting on Wednesday morning, although it was unclear if the militants were still in the city or had escaped into nearby mountains and forests that they have long used as hideouts.
Authorities did not give any updates on the whereabouts of Hapilon.
The Abu Sayyaf, based on the most southern islands of Mindanao, has kidnapped hundreds of Filipinos and foreigners since the early 1990s to extract ransoms. The United States lists it as a terrorist organisation.
Security analysts say Hapilon has been trying to unite Filipino militant groups that have professed allegiance to IS.
These include the Maute group, which is based near Marawi.
The Maute group has engaged in repeated deadly battles with the military over the past year.
Muslim rebels have been waging a rebellion since the 1970s for an independent or autonomous homeland in Mindanao, with the conflict claiming more than 130,000 lives.
The main Muslim rebel groups are involved in peace talks with the government.
But the Abu Sayyaf, Maute and other hardline groups are claiming they want to set up an Islamic caliphate in the south for IS, according to security analysts.
The US and other Western governments warned this month that terrorists were planning to kidnap foreigners in tourist hotspots in the western and central Philippines, adding to longstanding advisories of abduction threats in Mindanao.
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What happened to trigger martial law
Duterte acted after security forces on Tuesday battled dozens of gunmen in Marawi, a city of about 200,000 people. One policeman and two soldiers died in the fighting, authorities said.
Tuesday's violence was the latest in a series of deadly clashes with militants who have pledged allegiance to the Islamic State (IS) group, and are based throughout Mindanao.
Duterte's declaration fulfilled an often-repeated promise to impose martial law to combat the IS-linked militants, whom he has said are a growing threat to national security.
But he has said many times that martial law may be needed to solve many other problems, including illegal drugs.
Who are the militants
Authorities said they were battling members of two of the best-known groups to have declared allegiance to IS, the Abu Sayyaf and Maute organisations.
The military said it was hunting Isnilon Hapilon, an Abu Sayyaf leader who has been named the Philippine head of IS. The US government is offering a $5-million bounty for his capture.
The Abu Sayyaf is infamous for kidnapping foreigners and killing them if ransoms are not paid. It is also blamed for the nation's worst terror attacks, including the 2004 bombing of a ferry in Manila Bay that claimed more than 100 lives.
The Jakarta-based Institute for Policy Analysis of Conflict said last year that deepening cooperation among the Maute gang, the Abu Sayyaf and other pro-IS groups meant more deadly violence was "a matter of when, not if".
Is martial law necessary
Duterte had repeatedly said the growing influence of Islamic State was one of the nation's top security concerns, and martial law was necessary to stop it.
However Islamist militancy is not new to the southern Philippines, where a decades-long Muslim separatist insurgency claimed more than 120,000 lives.
Muslim rebels orchestrated a siege in the southern city of Zamboanga in 2013 that left more than 200 people dead, but the government of then-president Benigno Aquino did not declare martial law.
Aquino also said he had considered imposing martial law just before standing down last year in Sulu, island strongholds of the Abu Sayyaf in the far south of Mindanao.
But Aquino said he decided against it partly because military rule could spark resentment among local people.
What will happen under martial law
Martial law allows the president to "call out the armed forces to prevent or suppress lawless violence, invasion or rebellion", according to the constitution.
Defence Secretary Delfin Lorenzana said security forces would be able to arrest suspected militants and hold them for three days without charge.
Critics fear martial law will give security forces cover to carry out human rights abuses, citing the experience under dictator Ferdinand Marcos a generation ago.
During the nine years of martial law under Marcos, police and troops tortured, abducted and killed thousands of people who were critical of the dictatorship, according to rights groups and historians.
Duterte said Wednesday his version of martial law would be "harsh" and similar to that under Marcos.
What happens next
The constitution limits martial law to 60 days unless congress agrees to extend it. Duterte warned Wednesday it could last for a year.
The post-Marcos constitution imposed safeguards on martial law, including the requirement for congress to approve its imposition and extension. The Supreme Court can also rule on its legality.
However Duterte said in January he would ignore the consitutional safeguards if necessary, and may impose martial law across the nation.
"I don't care about the Supreme Court... because the right to preserve one's life and my nation, my country transcends everything else," he said.
