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.

Oscar Albayalde, police chief for the Manila region, said five drug dealers were killed on Sunday in a gun battle with police in a shanty town near a mosque near the presidential palace.

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

Four guns and 200 grams of crystal methamphetamine were recovered. Three others 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.

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.’

In the north of the main island of Luzon, drug enforcement agents and police seized a shipment of 180 kg (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.

On Sunday, the Maoist-led New People's Army rebels issued a statement supporting Duterte's all-out war against drugs, saying it might conduct its own drug operations against soldiers, police and local officials.