"September 21 is not a holiday. I have declared it as a Day of Protest. All those who want to protest against the government, the police, everyone… you go down and we will protest," Philippine President Rodrigo Duterte confidently dared his opponents ahead of massive protests organised against his controversial rule.

As many as 20,000 anti-Duterte protesters, composed of leftist-communist blocs, liberal civil society groups, and individuals from all walks of life, participated in separate massive rallies across Manila. They eventually converged in the Luneta Park, where the statue of 19th-century national hero Jose Rizal has stood as an inspiration for freedom fighters throughout the decades.

For Duterte's critics, it was nothing short of a Day of Rage, as protesters called for an end to Duterte's heavy-handed crackdown on illegal drugs and declaration of Martial Law in the southern island of Mindanao, where the government is fighting against Islamic State of Iraq and the Levant (ISIL also known as ISIS) affiliated groups.

For more than a year, Duterte has enjoyed almost unconditional support from a vast majority of Filipinos as he waged a shock and awe campaign against illegal drugs and criminality.

Yet, in recent months, the mood has gradually soured amid the gruesome death of minors, especially a teenager named Kian delos Santos, who was the victim of suspected extrajudicial killing at the hands of law enforcers. More than 50 children have reportedly been killed during Duterte's drug war, a heart-wrenching trend that has enraged a growing section of the society.

Specter of Martial Law

September 21 is a particularly evocative day in Philippine history. Back in 1971, the late Filipino dictator Ferdinand Marcos declared Martial Law on this day, placing the country under emergency powers, which was followed by death and torture of thousands of political opponents and activists.

Marcos reportedly tried to justify his Martial Law after a terror attack in Plaza Miranda in August of that year. The dictatorship lasted until 1986 amid massive public protests, later dubbed as "People Power" revolution.

READ MORE: Thousands of Filipinos slam Duterte's drug war

Fifteen years after, another Filipino president, Joseph Estrada, was toppled in a second "People Power" revolution, largely led by the urban middle class, who were enraged by the populist president's corruption and misbehavior in office. Today, Duterte's critics claim that he is an eerily familiar melange of Marcos and Estrada - a populist autocrat, who is bent on undermining the country's hard-fought democratic institutions.

The opposition Liberal Party, the Catholic Church, and civil society groups have also stepped up their pressure on Duterte, calling for an end to his bloody campaign against drug suspects.

Days ahead of the Martial Law commemoration, Duterte's Spokesman Ernesto Abella nonchalantly suggested that Duterte may declare nationwide Martial Law if the September 21 protests lead "to anarchy and disrupts the civilian government."

Earlier, Defence Secretary Delfin Lorenzana warned that "if the Left [communists] will try to have a massive protest, they will start burning (things) on the streets, they will disrupt the country, then [we] might [declare Martial Law]" nationwide.

Although General Lorenzana, who oversees the implementation of Martial Law in Mindanao where government forces are battling ISIL-affiliated elements, clarified that "that is very remote [option] to happen right now". Yet, Duterte's critics accuse him of gradually drawing the curtain on Philippine democracy in favour of autocratic rule.

Populist mobilisation

In a typically boastful manner, Duterte issued a challenge to his critics: "If you think three days [of protests], one month will do, go ahead, be my guest…I can live with it for about one year."

To demonstrate his political prowess, the populist Duterte mobilised thousands of his own supporters during the commemoration of the Martial Law, which he dubbed as the "National Day of Protest."

Though welcoming protests as a legitimate expression of democracy, the president has sought to dismiss and denigrate his critics by portraying them as part of a broader conspiracy to topple his government.

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To deflect blame, Duterte has portrayed the extrajudicial killing of minors as part of a "sabotage" effort led by a nefarious cabal of opposition members and criminal networks. He claims that the ultimate aim is to undercut his anti-drug campaign and, if possible, even end his presidency.

The president expected large opposition protests, especially with the well-organised communists ending their de facto coalition with the administration and decrying Duterte as a "dictator". The ugly political divorce came after growing disagreement over key policy issues between the president and his leftist-progressive allies, many of whom have been gradually eased out of top positions in government.

The opposition Liberal Party, the Catholic Church, and civil society groups have also stepped up their pressure on Duterte, calling for an end to his bloody campaign against drug suspects. For them, the country risks sliding into anarchy unless the president adopts a more humane and calibrated approach to the drug menace in the country.

In fact, the Philippines topped the 2017 Global Impunity Index (GII), which measures the frequency of extrajudicial killings and weakness of the justice system in countries around the world. There were also simultaneous anti-government rallies all across the country, including in Duterte's hometown of Davao.

Though the rallies were largely peaceful with no major outbreak of violence, the political fault lines have been sharpening in recent months. The Philippine democracy is looking more fragile and polarised than any time in recent memory.

Richard Javad Heydarian is a specialist in Asian geopolitical/economic affairs. He is the author of Asia's New Battlefield: The USA, China, and the Struggle for the Western Pacificand The Rise of Duterte: A Populist Revolt against Elite Democracy.

The views expressed in this article are the author's own and do not necessarily reflect Al Jazeera's editorial policy.