It started with reports of an electrical fault on the Tube and ended in death and disorder.
First reports of the co-ordinated London bombings only hinted at the chaos that unfolded around the tail end of morning rush hour in the centre of the city.
Emergency services rushed to the Aldgate East underground station where police reported one incident at 8.59am local time (7.59pm NZT).
There were reports of several people being wounded in the underground blasts, with one eyewitness telling Sky News he saw "bodies on the line".
British Transport Police confirmed there were "walking wounded" and said paramedics had responded to one report of a person classed as "life at risk".
"It's chaos, with people trying to work out what has happened," said a spokesman.
A spokeswoman for London's Metropolitan Police said: "There have been some casualties and this has been declared as a major incident."
The entire system was shut down with major thoroughfares blocked off by police and ambulance services.
Only then did panic begin to grip.
Television news reporters told of people frantically trying to reach loved ones by cellphones, and of others being reunited in tears.
One witness said the packed underground train he was on was 200m from King's Cross when there was a huge flash.
The train stopped and people were using umbrellas to smash windows of the trains. Passengers were led out of the train by underground staff.
He added people were milling around King's Cross covered in soot.
A witness at the Russell Square blast said the entire top deck of a bus was destroyed.
"I was on the bus in front and heard an incredible bang, I turned round and half the double decker bus was in the air," Belinda Seabrook told the Press Association. She said the bus was packed with people.
"It was a massive explosion ... there were papers and half a bus flying through the air," she said.
There was at least one body under a sheet on the pavement.
Scotland Yard said police were attempting to determine what had happened.
Clare Benson, 33, a city banker from Kensington, was on a tube at Edgeware Rd where the second incident took place.
"I was in the last carriage at the back of the train nearest the tunnel when I heard a huge bang - you could feel it," she told Sky News.
"The drivers got out and were looking into the tunnel, then they announced that we should evacuate. My ears are still ringing and I was shaking."
Police evacuated several mainline railway and underground stations after a blast near Liverpool St station, and a separate incident in the west of the capital.
"We believe there was some sort of explosion. There are some walking wounded at Aldgate," a police spokesman said, referring to Liverpool St.
A spokesman for London underground said another "incident" had taken place at Edgware Rd station in west London.
Earlier, British Transport Police said power surges had caused explosions.
One passenger, who had left the tube at Fenchurch St Station and walked to Aldgate East, told BBC Five Live that he saw injured people.
"As I walked through the bus station I could see people lying on the ground, black, as if they'd been covered in smoke. There were about three or four people on the floor being treated."
One of the evacuated stations was Stratford, which will be a key transport link for the 2012 Olympic Games.
Jim Millington told BBC World that he had just walked down from work and heard a bomb go off outside the British Medical Association Building in Russell Square. He said he arrived on the scene five minutes later.
"There were definitely fatalities."
"There were people streaming out of Aldgate station covered in blood," said Kate Heywood, 27, on her way to work. "There are shards of glass there, it is chaos."
Police sealed off large areas around other underground and mainline rail stations. Firemen donned chemical protection suits before rushing into stations.
Half a dozen people with soot-blackened faces sat on the floor at Russell Square underground station or stood in shock as police cordoned off the area, one witness said.