A series of murders in Marseille has sparked a political row over the growing turf wars between drug gangs who are terrorising poor housing estates, where one recent shooting victim was only 14 years old.

Dominique Laurens, the public prosecutor in Marseille, warned this week that France’s second-biggest city was facing “a very difficult moment” as she detailed an explosion in violence and score-settling killings that were defined by “extreme cruelty and a complete lack of humanity”.

Three men were killed in the Mediterranean port city this weekend in score-settling linked to the local drug trade, which is believed to bring in profits of millions of euros per month to competing cartels.

Marseille stands out from other cities such as Paris or Lyon because of its prevalence of Kalashnikovs and automatic weapons and the fact that drug-related killings and violent attacks often take place in public in order to spread fear among young people on the estates.

In the first shootout last weekend, two men aged 25 and 26, who were known to police, were shot with at least one automatic weapon as they talked on the street on an estate on Saturday night. Less than an hour later, a 27-year-old man, also known to police, was attacked by two men in balaclavas dressed in black nearer the centre of the city and forced into the boot of a car.

Laurens said the car was later set alight and the man burned alive. The prosecutor said: “The autopsy showed he was alive at the moment the car was set on fire, he had inhaled fumes and soot which led to his death.”

Last Wednesday, in broad daylight, a 14-year-old boy, Rayanne, was shot dead in the back by two people on a motorbike armed with at least one Kalashnikov-style assault rifle. He and a friend had been going for a sandwich on one of the city’s poorest housing estates. Another 14-year-old and an eight-year-old were hurt in the attack.

Laurens said: “We have never had victims so young.” She said there had been an “acceleration” in score-settling killings in Marseille since mid-June, with 15 people killed in turf wars since the start of the year. Dozens more investigations are under way into murders and attempted murders in the city.

The aunt of the murdered 14-year-old told BFM TV that “he was in the wrong place at the wrong time” and had not – like many teenagers – been made to work as a lookout for dealers on the estate.

Nathalie Roche, a magistrate and former children’s judge in the city, told French radio that young people were often forced to stand guard for drug dealers and were increasingly drawn into the violence, including “torture and kidnapping”, as a way for drug gangs to strike fear into communities.

With the Socialists in Marseille calling for more investment in the local justice system and the rightwing opposition warning of growing crime and insecurity, the French president, Emmanuel Macron, is expected to visit the city next week and announce increased funding to renovate schools and boost education projects.

Drugs gangs are seen to have filled an economic void in the de-industrialised city, where some tower blocks in the northern quarters have 70% youth unemployment and school drop-out rates are high.

Benoit Payan, the Socialist mayor of Marseille, said drug-dealing yielded millions in profits for cartels. “We’re talking about people who are obtaining heavy weaponry over the internet,” he added.

Samia Ghali, a Socialist deputy mayor in Marseille, warned that young people “are being exposed to extreme violence that even the worst fiction wouldn’t dare write”.

The interior minister, Gerald Darmanin, said the state was investing heavily in policing and anti-drug operations in Marseille.