Peter Larsson said the violence in the city of Gothenburg – a hotspot for migrants – is in danger of “boiling over” in the coming months.
The "criminal" gangs do not fear authority and they are rarely punished by officials due to overstretched services, he added.
He said: “Sweden is extremely naive, giving youngsters approval at the wrong age so they can’t be punished [if they commit a crime].
“When we arrest them, they are released without punishment. It is almost laughable."
The officer revealed that as many as 24 “unsolved gang murders, shootings, aggravated assaults" have built up because of tensions on the streets.
Tensions have soared in Sweden this month over the migrant crisis – with support for EU membership plunging.
Experts believe the ongoing refugee crisis to be the main reason for the waning support in the 28-member bloc.
Sweden received more than 160,000 asylum applications last year – the biggest influx in the EU as a proportion of the population
But earlier this year it emerged that around 80,000 could be removed from the country against their will, according to official estimates.