They came to kill her children one by one. First was Richard in 2001, then his brother Christopher. Bobby was taken from her the following year, and Fernando in 2007. Now Clarita Alia lives in fear that Arnold, her remaining son, is next. And far from protecting her shattered family, it is the police who are behind the killings, she says.

“The police said, ‘We will take your sons one by one’,” recalls the 54-year-old grandmother at the graveside of her murdered brood in the southern Philippine city of Davao. “They may kill me too, but I am not afraid to die. I’m already old.”

Insects hum in the humid tranquillity of this pauper’s graveyard. Below, the city of 1.3 million people, a tourist hub for some of the most spectacular scenery in south-east Asia, sprawls toward the Pacific Ocean. Mayor Rodrigo Duterte boasts that he has made this the safest urban zone in the country, but Davao’s motto, “love, peace and progress”, is belied by a killing spree that has claimed nearly 900 lives, including dozens of children.

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The mayor of the country’s second-biggest city says they all deserved to die. “What I want to do it so instil fear,” he told reporters earlier this year. “If you are doing an illegal activity in my city, if you are a criminal or part of a syndicate that preys on the innocent people of the city, for as long as I am the mayor, you are a legitimate target of assassination.”

Condemnation and press coverage have failed to stop summary executions of what Mayor Duterte calls “society’s garbage”: alleged petty drug dealers, young toughs and street children. Vigilantes have murdered 894 people in the last decade, including at least 80 minors, according to the Tambayan Center for Children’s Rights, a Christian NGO in the city centre. The youngest victim was just 12.

And the pace of killings is increasing: 57 people were stabbed or shot in the first three months of this year, up from two in the whole of 1998. The killers escape arrest thanks to the tolerance and sometimes “outright support of the local authorities”, says a new report on the killings of Davao’s Death Squads by the US-based Human Rights Watch. “Impunity for such crimes is almost total,” it adds.

Most chillingly of all, officially-sanctioned vigilantism is spreading as the Phillipines’s economic woes deepen, warns Edith Casiple, Tambayan’s executive director. “The problem is now all over the country. Other leaders are copying Mayor Duterte.” In several cities, including the capital Manila, politicians have praised Davao’s rough justice. Activists, trade unionists and newspaper reporters have been added to the assassins’ lists; only Iraq is more dangerous for journalists, says media watchdog Reporters Without Borders.

Police officers in collusion with city governments across the southern island of Mindanao are involved in the targeted killings – known in the local press as “salvagings” and “rub-outs”, say human rights groups.

Executions have also been reported in the troubled holiday resort of Cebu and in Manila. Philippine President Gloria Macapagal Arroyo, who once appointed Duterte as an advisor on peace and order, has “largely turned a blind eye” to the murders, claims Human Rights Watch.

Most executions follow a similar pattern. Police officers or government officials from barangays – local administrations – approach alleged troublemakers to warn them they have made a hit list known as the “order of battle” – or, in Davao, “Duterte’s list”. Failure to heed the warning by quitting “illegal activities” or leaving town is a sentence of death, usually carried out by men on motorbikes carrying butchers’ knives or .45-calibre handguns.

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The vigilantes have achieved their purpose: instilling terror in the slums, says Renante Ventula, 25, who lives on Davao’s streets. “When we talk about them, it’s in low voices because we don’t now who is listening.” Last October, he says his friend was murdered in an internet cafe. “He had been warned by the police. Two men arrived on motorbikes without license plates, went into the cafe and took turns stabbing him. He had 10 stab wounds.” When the cops arrive at the scene of such executions, they ignore investigative procedure and fail to interview witnesses, claim observers.

Few of the killers have been caught and not one of the murders has been condemned by the mayor, who used to read out the names of “troublemakers” on his weekly TV show. The vigilantes, who are sometimes recruited from the ranks of petty criminals, are guided by police officers or ex-police, says Human Rights Watch. Killings are often subcontracted; by the time they reach the street, they can reportedly be done for as little as 350 pesos (£4.60).

Police spokesman Doble Rogelio denies the claims. “We are trying our best but there are no witnesses to the killings,” he said, the now standard police response to accusations that they are not doing enough to catch the culprits. The San Pedro station where he works is festooned in religious imagery: a portrait of Jesus Christ adorns the hallway and a huge placard quoting the prophet Jeremiah stands at the entrance. Officer Rogelio says he has heard of Clarita Alia’s murdered children. “The Alia family was a notorious clan, frequently engaged in rioting. They were apprehended many times. Perhaps their mother has hatred toward the police.”

Local businesses claim the murders have made Davao a safer destination for the thousands of tourists who visit it. “Duterte is very popular here,” explains Emily Lawas-Juausengpue, a restaurant owner. “If you are not a criminal, you have nothing to fear.” The mayor has been elected four times and has been dubbed “Dirty Harry” in some of the Filipino press. Photographers often snap him holding a shotgun or sitting on the motorbike he rides to work on.

But Irish priest Shay Cullen, who has lived in the Philippines for three decades, reacts angrily to suggestions that the victims deserved to die. “These are not criminals. They’re children who have never been convicted of anything.” A decade ago, Father Cullen was sued for libel by Davao’s previous mayor after he started a letter-writing campaign to halt the death squads. “It was a scary time because I could have been assassinated myself.” He says the growing annual death toll shows people like Mayor Duterte are now “out of control” and protected by powerful political backers. “Who can prosecute him? Only the World Court.”

Years of pressure by campaigners like Father Cullen seemed to pay off this year when the Philippine Commission on Human Rights held a three-day hearing in March on the Davao killings. “Children are being executed,” Chairperson Leila DeLima told the press. Most alarming, “is the growing culture or mentality of acceptance of the executions...this is worse than apathy and indifference,” she warned, calling them “selective vigilantism” that targeted the poor and left big-time criminals untouched.

Mayor Duterte lived up to his gung-ho image at the hearing. Asked by DeLima what he would do if he captured a member of the shadowy death squads operating in his city, he intoned: “If I catch you killing someone, I will shoot you in front of the people”. The commission found him accountable for the murders on his watch and the mayor quit his supervisory role over the police force in April, but most observers believe he is still the city’s political kingpin.

“The hearings were just a game by the government, to show that something is being done,” spits Clarita Alia, who testified to the commission about her experiences. In the rancid one-room hovel where she lives, oldest son Arnold dozes on the only bed, next to her baby grandchild. She raised her seven children here in the teeming city markets of Bankerohan, where she sells cigarettes on the street for a living. None of her children finished primary school and some drifted into petty crime. When the police came for Richard, they say he had already been arrested ten times. “I fought with them,” she recalls. “They didn’t have a warrant and I told them they couldn’t take my son. And the senior policeman said: ‘OK, watch out because your sons will be killed, one by one.’”

Richard was stabbed to death a few weeks later, aged 18. Christopher, 17, and Bobby, 14, were knifed within months of each other. In desperation, she sent Fernando to live out of the city but after he returned, he was killed by the faceless assassins in April 2007, aged 15. Only Arnold remains, until the killers come again. “It hurts every time I talk about it,” she says. “But I don’t have a choice because I want the world to know what happened to my children.”