Police killed at least 28 people overnight in a crackdown in the Philippines capital Manila, authorities said on Thursday, a second night of heavy bloodshed this week in an intensification of President Rodrigo Duterte's fierce war on drugs and crime.

The killings across Manila followed 32 deaths in police raids on Monday night in Bulacan province, which borders the capital. Together, they mark the deadliest period of a drugs-focused crackdown that has killed thousands of Filipinos, and caused international alarm, since Duterte took office over a year ago.

Colonel Erwin Margarejo, spokesman for Manila police, described the raids that started late Wednesday in Manila as "one-time, big-time" operations, the same term used by police in Bulacan, who said the victims died because they chose to put up a fight.

The term has been used by Philippines police to describe a coordinated anti-crime drive in crime-prone districts, usually slums or low-income neighbourhoods, often with additional police deployed.

It was however not immediately clear what was behind the step-up in the number of coordinated police operations this week.

According to police reports, a total of 223 people were arrested in Manila and Bulacan. The reports said police launched 84 operations in the two regions, the majority of which were "buy-bust" stings, in which plain-clothes officers attempt to trap drug peddlers.

There were no reports of any police casualties.

"The president did not instruct me to kill and kill," national police chief Ronald dela Rosa told reporters. "I also don't have any instructions to my men to kill and kill. But the instruction coming from the president is very clear that our war on drugs is unrelenting. Those who were killed fought back."

Duterte unleashed his crackdown the day he took office on June 30 last year after a convincing win in an election in which he campaigned heavily on a promise to use deadly force to wipe out crime and drugs.

On Wednesday, he indicated the latest operations had his blessing.

Duterte said it was good that 32 criminals had been killed in Bulacan, then added: "Let's kill another 32 every day. Maybe we can reduce what ails this country."

On Thursday, he said he would not just pardon police officers who killed drug offenders during the anti-narcotics campaign, but also promote them.

Chito Gascon, the chairman of the Philippine Commission on Human Rights, said Duterte's comments emboldened police "to do their worst."

"The police are essentially free to do what they will because they are almost guaranteed that they will not be investigated or charged," Gascon said.

"OUTRAGED"

Vice President Leni Robredo, who is from a political party opposed to Duterte, sharply criticised the killings and said it was "something to be outraged about".

"We are not like that," she said in a statement. "This is not us. We have long condemned the culture of impunity. Let us not allow it to return."

Manila police chief Oscar Albayalde said there had been no instruction to change or increase the scale and scope of the anti-drugs campaign.

"This is just part of our 'one-time, big-time' operations against illegal drugs," he told Reuters.

Duterte also chided human rights groups on Wednesday for getting in the way of his anti-drugs campaign and said police should shoot them if they obstructed justice, a remark the New York-based Human Rights Watch said puts activists "in grave danger".

Its deputy Asia director, Phelim Kine, described the comments as "like painting a target on the backs of courageous people working to protect the rights and upholding the dignity of all Filipinos."

The exact number of people killed during the war on drugs is difficult to quantify, with no independent statistics available and police providing comprehensive data only for deaths during anti-drugs operations, where official accounts typically say suspects resisted arrest.

From the start of the drugs war to the end of July, police said over 3,400 people were killed in their operations. Police said about 2,100 deaths among some 13,500 murders over the same period were drugs-related, attributed to turf wars, informants being silenced, or vigilantes killing drug users.

Most of the people killed have been drug users or small-time dealers in poor communities, .

A total of 65 policemen have been killed on the job during this time.

Critics maintain that members of the Philippine National Police (PNP) are executing suspects and say it is likely they have a hand in thousands of unsolved murders of drug users by mysterious vigilantes. The PNP and government reject that.

Although the violence has been criticised by much of the international community, Filipinos largely support the campaign and domestic opposition to it has been muted.

Several Senate hearings into allegations that Duterte operated a death squad when he was a city mayor and was now using the same approach on a national scale have been inconclusive, while an impeachment complaint filed earlier this year was dismissed by Congress.