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Thirty "drug dealers" have been killed since Rodrigo Duterte was sworn in as Philippine president on Thursday, police said, announcing the seizure of nearly $20 million worth of narcotics but sparking anger from a lawyers' group.

Duterte won the election in May on a platform of crushing crime, but his incendiary rhetoric and advocacy of extrajudicial killings have alarmed many who hear echoes of the country's authoritarian past. He has been nicknamed "The Punisher."

Oscar Albayalde, police chief for the Manila region, said five drug dealers were killed Sunday in a gunbattle with police in a shanty town near a mosque not far from the presidential palace.

Filipino funeral workers carry a body following an anti-drug operation by police in Pasay City, south of Manila, on Tuesday. MARK R. CRISTINO / EPA

"My men were about to serve arrest warrants when shots rang out from one of the houses in the area," Albayalde told reporters, saying officers returned fire and killed five men.

Four guns and 200 grams of crystal methamphetamine were recovered. Three other "drug dealers" were killed in other areas in Manila on Sunday and 22 were killed in four areas outside the capital.

More than 100 people have died, mostly suspected drug dealers, rapists and car thieves in stepped up anti-crime police operations since the election on May 9.

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Edre Olalia, secretary-general of the National Union of People's Lawyers, said the killings must be halted.

"The drug menace must stop ... Yet the apparent serial summary executions of alleged street drug users or petty drug lords which appear sudden, too contrived and predictable must also stop," he said in a statement. "The two are not incompatible."

Philippines President Rodrigo Duterte walks past military cadets on Friday. ERIK DE CASTRO / Reuters

In the north of the main island of Luzon, drug enforcement agents and police seized a shipment of 400 lb of "shabu" (methamphetamine) worth about 900 million pesos ($19.23 million) from either China or Taiwan, national police chief Ronald dela Rosa said.

The shipment was unloaded at sea and brought to shore by small fishing boats before delivery to Manila's Chinatown, he said.

Duterte tapped into voters' disgust with the Philippines' political elite and the failure of successive governments to tackle poverty and inequality, drawing comparisons with Donald Trump and the rise of assertive populists across the globe.

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In his maiden speech last week, the president conceded that many believe his methods "are unorthodox and verge on the illegal." However, the 71-year-old former prosecutor said he knew right from wrong and would abide by the rule of law.

Duterte was mayor for 22 years of the southern city of Davao, where, according to human rights groups, death squads have killed at least 1,400 people since 1998, most of them drug-pushers, addicts, petty criminals and street children.

He denies any involvement in vigilante killings.