The teenager hit by a tram during Ride The Lights in Blackpool had to have surgery after being horrifically injured, police said.
The 14-year-old was taken to Alder Hey Children's Hospital in Merseyside after being knocked off his bike and trapped under the Bombardier tram opposite the Queen's Hotel in South Shore on Tuesday night.
Police yesterday said he was being treated for serious hip, pelvic, and leg injuries, while a spokesman this morning confirmed the boy, from Blackpool, had to have surgery.
The Prom was shut while resort firefighters, who are specially trained in tram rescues, freed the boy at around 9.50pm alongside colleagues from the Chorley-based urban search and rescue team.
A joint investigation between Lancashire Police and the Rail Accident Investigation Branch (RAIB) remains underway, with resort chiefs awaiting the outcome before deciding whether or not trams will run at next year's Ride The Lights.
CCTV footage captured from the cab of the tram involved, which had around 90 passengers on board, is expected to play a key role.
The popular annual free event, attended by thousands, sees the Prom shut to road traffic so families can cycle through the Illuminations.
Ahead of this year's Ride The Lights, which ran from 7pm to 10pm, police and tourism chiefs had warned that trams could pose a danger, and said bikers should "be aware that the tramway will be operation".
Council leader Simon Blackburn said: "We will be giving our full co-operation to the investigation ... to determine the circumstances surrounding the accident.
"Ride The Lights is a long-standing event, organised with a meticulous degree of planning to ensure all of the participants can enjoy a safe, car-free environment."
One passenger on the tram involved fell into another when it stopped suddenly. Both were checked over by paramedics but did not need treatment.
Tram services resumed for the final service at 12.30am on Wednesday, with the tram involved due to be inspected for damage before heading back out onto the tracks yesterday.
Jane Cole, the managing director at Blackpool Transport, said: "First and foremost, our thoughts are with the boy and his family. On behalf of Blackpool Transport, I would like to send him our best wishes and also our thanks to all the emergency services who assisted."
Ms Cole said her staff would co-operate "fully" with the investigation "to understand how the incident happened and if it could have been prevented in any way".