The 7/7 bus explosion sent a massive "chimney" of grey smoke billowing into the air, laden with debris and human body parts, an inquest heard today.
The thick plume soared up to around 200 feet above the red double-decker after teenage terrorist Hasib Hussain detonated his home-made device.
As the dust settled, police officers who witnessed the blast distinguished a pair of legs protruding from the wreckage while desperate victims cried out for help.
The inquest also heard how a box marked "microwave" stored in a luggage rack sparked security fears and led to a search of the bus with sniffer dogs - a task which lasted several minutes and was conducted as horrifically injured passengers lay dying.
But the item, brought on by Gladys Wundowa, 50, from Ilford, Essex, proved to be innocuous.
Ms Wundowa, who worked as a cleaner at University College London, was among 13 people killed by the blast at Tavistock Square.
Recalling the scene, British Transport Police (BTP) Constable Gary Sims said: "There was a very dark grey-white smoke cloud which went possibly 100 feet to 200 feet up in the air and as that went up ... there was a lot of debris and there were unfortunately parts of people in that.
"At the same time the two sides came out from the rear of the bus. Areas came out and curled towards us and at that point the roof came off."
This landed in front of the bus, he told the inquest.
His colleague, BTP Inspector Ciaran Dermody, told the hearing how the shocked pair, along with fellow officer Pc Neville Lazenby, watched from an unmarked police car as the top of the bus appeared to open out before them and thick smoke rose high above it.
"It went up out of the bus almost like a chimney," he said.
The group stared in horror as the roof then "flopped" down.
"We all in the car, I think, said at the same time, there's a bomb on the bus," he said.
"We knew instantly it was a bomb, we didn't think it was anything else."
Racing towards the wreckage, they found wounded passengers lying scattered on the road and on the pavement.
"The bus seemed to be compressed down to my height which is 6ft 1in," Insp Dermody told the hearing at the Royal Courts of Justice in central London.
"(The bus) was compressed down and there were a number of people on that top deck of the bus trapped."
Hussain, the youngest of the four suicide attackers, detonated his device nearly an hour after explosions ripped through three Tube trains on July 7, 2005.
As well as killing themselves and 52 others, the bombers injured more than 700 people.