PARIS (Reuters) - Violence spread in Paris’s northern suburbs for a fourth night and French police arrested a dozen people, police said on Wednesday, amid accusations that police officers raped and beat a man they were detaining.

Dozens of vehicles and a nursery school were set on fire by youths during standoffs with police in an area of Paris where riots in 2005 drew global attention to the stark contrast between wealthy Paris and the suburbs that surround it.

As well as damaging the nursery school and a car sales outlet, youths had also used a shopping trolley full of petrol bombs, police said.

The trouble began in Aulnay-sous-Bois on Feb. 2 where four police officers were accused of using excessive force while arresting a 22-year-old man, including raping him with a baton.

“For the moment we’re talking of very violent but isolated standoffs,” said Luc Poignant from the SGP police union.

While much more limited than 12 years ago, the unrest served as a reminder of the simmering tensions in neighborhoods with higher-than-average unemployment and big immigrant populations, as France prepares to elect a new president this year.

The jobless rate in Aulnay-sous-Bois is nearly twice the national average of 10 percent.

“Sadly these neighborhoods have been turned into ghettos,” said police representative Yves Lefebvre, who said that police were not sufficiently trained or equipped to deal with the problems in the sprawling suburbs where drug dealing is rife.

The four police officers have been suspended pending an inquiry and one has been placed under formal investigation for suspected rape and three others for unnecessary violence.

The victim, a young black man, has himself called for calm, and he and his family say they trust the justice system will deal properly with the affair.

President Francois Hollande visited the man on Tuesday at the Aulnay hospital.

Police said the skirmishes on Tuesday night were mainly in towns around Aulnay-sous-Bois, which itself was relatively calm.

The unrest is playing out against a backdrop of growing political uncertainty in France, with support growing for far-right leader Marine Le Pen and conservative Francois Fillon hit by accusations his wife was paid by the state for a fake job.

The 2005 riots, in which 10,000 cars and 300 buildings were set on fire, prompted then interior minister, Nicolas Sarkozy, to declare a state of emergency.

Political opponents say Sarkozy made matters worse when, as president from 2007 to 2012, he scrapped specialized local police teams and cut police staffing by 10,000.