Italy was in shock Thursday after the dramatic rescue of 51 children taken hostage by their school bus driver who torched the vehicle in protest at Mediterranean migrant deaths.

The Italian driver of Senegalese origin on Wednesday hijacked the bus as it was taking the 12-13 year-olds from a gym to school in Crema, east of Milan.

Armed with two petrol canisters and a cigarette lighter, Ousseynou Sy threatened the youngsters, took their telephones and told the adults to tie them up with electric cable.

"He blocked all the doors with chains," teaching assistant Tiziana Magarini told AFP.

"He showed me a knife and told me to tie up all the children."

The 40-minute ordeal, during which the bus also slammed into a car, was brought to an end when police managed to smash windows open and get those onboard out just as the driver set fire to the vehicle.

A dozen children and two adults were taken to hospital for smoke and fume inhalation, according to emergency services.

"It's crazy, absurd, it's unacceptable. Someone has to pay, and dearly," said Filippo Razzini, the father of a pupil at the school in the small town of Crema who was not on the bus.

"It's good to go back to school today because unfortunately these things are today a reality. But if it were up to me I'd be out there waiting for this guy somewhere," he told AFP.

The driver's lawyer said his client had wanted to "draw attention to the consequences of (Italy's) migration policies".

Italy has clamped down on immigration under far-right Interior Minister Matteo Salvini, demanding it close its ports to charity vessels rescuing migrants who are trying to cross the Mediterranean.

"This villain has to pay for everything," said Salvini, whose League party is riding high in the polls in part because of its tough anti-migrant stance.

He said Sy's citizenship could be revoked if convicted of terror under a tough security decree introduced last year.

Conversely, the Moroccan- and Egyptian-born fathers of two of the boys who managed to call police voiced hope their Italian-born sons would now get Italian nationality.

'Lone wolf'
The Milan police anti-terrorism unit has been charged with investigating the hostage-taking, during which Sy reportedly told students: "No one is getting out of here alive."

The incident has shaken Crema's community.

"My daughter was in shock yesterday, she said 'Mummy, I could have been there too'," parent Luisa Ginelli told AFP Thursday.

The driver had no links with Islamic terrorism and "acted as a lone wolf", Alberto Nobili, head of counter-terrorism at the Milan public prosecutor's office, told a news conference.

Nobili said Thursday that Sy had planned the hijacking over several days and "wanted the whole world talking about his story".

He posted a video on YouTube to explain his actions and call on relatives and friends in Crema and Senegal to take action, saying: "Africa -- arise."

Sy got his Italian nationality and job in 2004 and managed to keep subsequent convictions for drink driving and sexual assault of a minor secret from his employer, the Corriere della Sera newspaper reported.

A neighbour told La Stampa newspaper that he was known as "Paolo".

"That's what we called him because his name was too complicated. I saw him go out every morning, he drove a bus. A quiet man but solitary," she said.

Colleagues told Italian media that Sy's separation from his Italian wife, with whom he has two teenage children, was "when his problems started".