Thousands of people have protested in the Philippines on the 45th anniversary of the declaration of martial law in the country while denouncing what they say are President Duterte's authoritarian tendencies and his bloody crackdown on illegal drugs.

Hundreds of riot police on Thursday were deployed in the capital, Manila, to secure the marches and rallies, among the largest against Rodrigo Duterte since he took office last year.

A group calling itself a "Movement Against Tyranny" used the anniversary to highlight human rights violations under Duterte's so-called drug war on drugs.

According to police statistics, more than 3,000 suspects have been killed in anti-drug operations since Duterte became president on June 30, 2016.

Another 2,000 more have died in drug-related homicides, according to government figures.

READ MORE: Thousands demand end to killings in Duterte's drug war

Waving red flags and carrying placards that read "Stop the killings", left-wing groups gathered in three areas of Manila, including a bridge leading to the presidential palace. They assembled later at a park by Manila Bay for the main protest.

Another group of protesters staged a separate rally at the Commission on Human Rights, which has been denounced repeatedly by Duterte for raising the alarm over his campaign against illegal drugs.

"Marcos, Duterte, no different, they both kill," the activists chanted, referring to the late leader Ferdinand Marcos who imposed military dictatorship for eight years in 1972, citing the threat of communism.

Marcos' martial law era, which ended in 1981, was marked by massive human rights violations and the muzzling of civil liberties.

Al Jazeera's Jamela Alindogan, reporting from the protest in Manila, said that survivors of the martial law period draw a lot of similarities between former president Marcos and the current rule of Duterte.

"The protesters are asking the president to stop his so-called war on drugs and instead focus on institutional reform, especially when it comes to the Philippine national police and the judiciary," she said.

Mindanao martial law

Duterte declared Thursday as a National Day of Protest to allow people "to express their grievances as part of their constitutional rights" as he suspended work in government offices and classes in public schools.

But he warned protesters not to break the law.

"I will not hesitate to use force even if it would mean my downfall as president of this country, remember that," Duterte said on Friday on state TV.

Several thousand demonstrators took the opportunity to gather separately to show their support for the president.

READ MORE: MPs back Duterte's bid to extend Mindanao martial law

Duterte imposed military rule in the southern Mindanao region for 60 days on May 23 when hundreds of ISIL-linked fighters occupied parts of Marawi city.

The Philippines Congress approved in July Duterte's request to extend martial law on the southern island until the end of the year.

Duterte has also threatened to impose nationwide martial law to combat the threat from fighters in the south.

Last week, Defence Secretary Delfin Lorenzana said Duterte might expand martial law from the southern Philippines to the entire country if the September 21 rallies turn violent.

But, on Thursday, Presidential spokesman Ernesto Abella tried to allay those fears.

"If they are saying that martial law is being planned, it is not so," he said in a radio interview.