The full horror at King's Cross unfolded over several hours — during a desperate battle to free maimed victims.

Bodies were trapped in the mangled wreckage while hundreds of terrified passengers fought for breath amid clouds of thick black smoke.

London's second coordinated terror blast at 8:56 a.m. Thursday tossed the Piccadilly Line (search) train around like a child's toy — and killed 21.

Rescuers were on Friday still battling to recover bodies — after they discovered the blast caused part of the tunnel to collapse. It was feared the death toll could rise by another 20.

The bomb ripped the packed train apart just 20 seconds into the journey from King's Cross (search) to Russell Square (search).

Inside the carriages nearest the bomb it was a scene of utter and bloody devastation.

Some passengers were killed outright, a few literally thrown from the train on to the track.

Others had arms or legs sliced off by flying shards of metal.

Everyone was plunged into darkness as acrid smoke then began billowing through the tunnel.

Desperate sounds of the dying and the terribly injured could be heard amid the shouts and screams of other passengers.

Outside, rescuers clambered along the tunnel, not knowing the horrific scenes that lay ahead.

Survivors — believing they could still die in a massive fire — were trapped for half an hour before workers managed to pry open doors of one of the carriages.

At King's Cross station, dozens of fire engines, police cars and ambulances crowded outside. Makeshift mortuaries were set up in nearby hotels as bodies began to piled at the side of the track underground.

Dazed and blackened passengers — the ones who could walk — were led along the tracks and out into the air at King's Cross.

Scenes above ground brought back a chilling reminder of the 1987 fire at the station, in which 31 people died.

At one point a priest was escorted on to the concourse to give last rites to the dying.

Survivor Andy Dolan, one of the last to squeeze on to the ill-fated train, emerged and said: "I can't believe how many people died. I'm so lucky to be alive. No more than 20 seconds after the train pulled away I heard what I can only describe as very dull explosion.

"At the same moment I felt an impact and the air seemed to buffet around me. The train came to a halt and the lights went out.

"The carriage filled up unbelievably quickly with rank, acrid smoke. The doors wouldn't open and everyone crouched down to try to get out of the smoke.

"Some people descended into hysterical panic. Others were desperately trying to calm them down."

Andy, 33, from Hitchin, Hertfordshire, added: "The first thought that ran through my head was, 'My God, was that a bomb?' My last thought was, 'I'm going to die.'"

Anna Pacey, above ground at King's Cross, said: "It was a scene of total devastation.

"There were just hundreds of people coming out with head injuries, cuts and burns.

"People were complaining of deafness in at least one ear, and others were helping carry the injured out.

"People had their hair, eyebrows and facial hair singed. Some were carrying people in blankets who had lost limbs."

Mary Burke, 50 from Sandistead, Surrey, said: "Suddenly there was a huge bang and I fell to the ground. Everything went dark. There was glass everywhere and I was thrown to the floor.

"I felt the impact on the side of my head and under my feet. I just thought 'I'm gone.' Everyone just panicked, everyone was screaming and crying. I really did not think I was going to get out of this alive."

Another victim — a top London medic — was due Friday to have his leg amputated after being caught in the blast.

Epilepsy specialist Prof. Philip Patsalos, 52, was rushed to the Royal London Hospital (search) — but his worried wife Ellie did not track him down until 9 p.m. Thursday night.

Ellie, of Winchmore Hill, north London, only found her injured husband after showing hospital staff his picture.

She said: "I showed the receptionist a photo of him and was taken to his bed.

"He looks badly scarred and was too dazed to talk. I don't know what the future holds for him now."

Bank worker Zeyned Basci, 21, of Edmonton, North London, emerged above ground splattered in blood.

She said: "There was blood everywhere. People had been sprayed with glass. One woman was unconscious on the floor. Her face was badly cut and gouged. All you could see was flesh and blood.

"We thought the carriage was going to catch fire. I really thought I was going to burn alive."

Many injured were ferried to the Royal Free Hospital (search) at Hampstead, North West London and the Royal London Hospital in the East End.

Some put their survival down to the mass of people on the train cushioning them from the explosion.

Fiona Trueman, 26, said after being treated: "We sort of cushioned each other during the blast because the compartment was so full."

Devastated dad George Kolias wept last night as he kept a vigil at the hospital bedside of daughter Danielle, 19, in intensive care.

Shaking with emotion, George, 45, said: "I can't believe what they have done to my little girl. It is hard to recognize her.

"She came round and struggled to talk to me. She started to cry and I felt my heart would break. How could anyone do something like this?"