MANILA — President Rodrigo Duterte of the Philippines is expected to sign sweeping antiterrorism legislation that critics said on Thursday would allow the authorities to classify government opponents as terrorists and detain people for critical social media posts.

The measure, which has passed both houses of Congress, neared finalization as the United Nations released a scathing report that cites widespread human rights violations under Mr. Duterte, including the extrajudicial killing of more than 8,000 people.

Despite years of international and domestic criticism over rights abuses, Mr. Duterte appears eager to double down on his strategy of suppressing dissent and to give the police an even freer hand to crack down.

Critics said the legislation was so broadly written that it would allow the arrest and detention of people without a warrant or a charge for criticizing the government or acts such as causing property damage or carrying a weapon.

“It’s obvious that the bill is not after real terrorists,” said Senator Leila de Lima, a critic of Mr. Duterte who has been imprisoned for more than three years. “There is a new crime here, called inciting to terrorism. Just protest against not receiving aid amid the pandemic, and they can charge you with ‘inciting.’”

Mr. Duterte, 75, won election in 2016 on a pledge to kill 100,000 criminals in his first six months in office and dump so many bodies in Manila Bay that the “fish will grow fat.” He is scheduled to step down in 2022 at the end of a six-year term.

The new legislation would create an antiterrorism council to be appointed by the president that would have the authority to designate individuals and organizations as terrorists. Anyone labeled a terrorist or suspected of belonging to a designated group could be detained without a warrant.

It would also classify many acts that are already crimes as terrorism, including engaging in acts that cause death, injury or property damage; making, possessing or transporting weapons; or any acts deemed to undermine public safety. Some crimes could bring heavier punishment if prosecuted as acts of terrorism.

The head of the National Union of People’s Lawyers, Edre Olalia, said the measure would undermine basic freedoms that were restored by the Filipino people in 1986 when they ousted the dictator Ferdinand Marcos in the People Power uprising after more than 13 years of martial law.

As president, Mr. Duterte has mused about placing the entire country under martial law and called Mr. Marcos his idol.

If the antiterrorism bill were to become law, it would give Mr. Duterte’s handpicked council the power to designate opponents as terrorists who could then be held without charges for up to 24 days, Mr. Olalia said.

“Critics, dissenters, opponents and even ordinary folks that ruffle the feathers of the powerful and favored can be arrested without a judicial warrant and mistakenly, wrongly or maliciously labeled a terrorist,” he said.

The head of the rights group Karapatan, Cristina Palabay, said social media posts were not singled out in the bill but would be covered if they were construed as inciting terrorism.

“While there is no specific provision, any article or post on social media that may be interpreted as acts of incitement to terrorism can be penalized,” she said. “And that is among the most dangerous provisions of the bill that infringes on freedom of expression and other fundamental rights.”

Ms. de Lima, a lawyer and former Philippine rights commissioner who is accused of corruption but whose case has never come to trial, has continued to speak out against Mr. Duterte from her jail cell.

“The tyrant-in-chief in Malacañang has been using the full weight of his office against me,” she said, referring to the presidential palace. “The antiterror bill has a profoundly chilling effect on our own fundamental freedoms of thought, of expression, of an independent press, and of maintaining association and peaceful assembly.”

Mr. Duterte has not commented publicly on the specifics of the bill but urged the House of Representatives on Monday to speed up its passage. The measure, which was approved by the Senate in February, was passed by the House on Wednesday evening.

The report released by the United Nations High Commission for Human Rights cited estimates that at least 8,663 people had been killed without due process in Mr. Duterte’s so-called war on drugs, and possibly three times that number. That includes at least 73 children, including one who was 5 months old.

In releasing the report, the United Nations human rights chief, Michelle Bachelet, said the Philippine police and government had not been held accountable for the vast majority of the killings.

“The report has documented deep-seated impunity for serious human rights violations, and victims have been deprived of justice for the killings of their loved ones,” she said. “Their testimonies are heartbreaking.”

In addition to the deaths of thousands of supposed drug suspects, the commission said it had verified the killing of at least 208 rights defenders, legal professionals, journalists and trade unionists between January 2015 and December 2019.

The report noted that Mr. Duterte’s government had moved to suppress dissent by seeking to put media outlets out of business — including the online news outlet Rappler and the major broadcaster ABS-CBN, which has been forced off the air — and by jailing opponents, such as Ms. de Lima.

The report also found fault with the government’s approach to the coronavirus for relying on “the same heavy-handed security approach.”

“While important measures were taken to mitigate the pandemic’s economic impact on vulnerable communities,” the report said, “threats of martial law, the use of force by security forces in enforcing quarantines, and the use of laws to stifle criticism have also marked the government’s response.”

The president’s spokesman, Harry Roque, denied that the government was using the pandemic as an excuse to clamp down on freedom of expression or tighten censorship. He asserted that the Philippines takes pride in protecting citizens’ rights.

“The Duterte administration takes each case, be it a violation of the freedom of the press or of any other human rights, brought before its attention seriously and resolves each one within our domestic processes,” he said.

Ms. Bachelet, a former president of Chile, said the Philippines faced many challenges, including poverty, armed conflict, frequent natural disasters and the pandemic. She urged the government to base its response to those challenges on respect for rights.

“People who use or sell drugs do not lose their human rights,” she said. “People who disagree with government policies and criticize them, including in international forums, should not be vilified as terrorist sympathizers.”

Jason Gutierrez reported from Manila, and Richard C. Paddock from Bangkok.