London has been hit by four explosions - three on underground trains and one on a bus. See below for more information on each location.
LIVERPOOL STREET TUBE STATION
Seven people are confirmed dead after an explosion rips through a carriage on an Underground train about 100 metres outside Liverpool Street station.
Emergency services are first called at 0851BST.
A passenger tells of an "enormous bang and a lot of smoke". He adds: "A group of us got fire extinguishers and were able to smash through the carriage door."
Passengers are rushed out of Liverpool Street station by police and Underground staff. Anxiety increases as mobile phones stop working, leaving people unable to contact friends and relatives.
At Liverpool Street Station in the City financial district, the wounded are treated by medics as they lie on the concourse.
A doctor at Aldgate estimates that at least 90 people had been wounded in the explosion.
Liverpool Street is a major underground and overland interchange through which hundreds of thousands of commuters, many of them financial workers, pass each day.
EDGWARE ROAD TUBE STATION
The explosion occurs at 0917BST on an Underground train at the station. It rips through a wall and affects two further trains.
Police later confirm that seven people have died.
After the blast passengers begin emerging from the station, many covered in blood and clearly distraught.
Paramedics go into the station, prompting fears there may still be injured people trapped underground.
A woman passenger tells the BBC there was a huge bang as two trains passed in a tunnel just outside Edgware Road. "All the lights went out and there was a lot of smoke. I believe it's the second carriage that has all the injured people in it."
A senior police official said there had been a number of fatalities but that "things are still relatively confused".
Those able to walk are looked after at a nearby hotel. Emergency services are described as "stretched".
Edgware Road is a busy station with four underground lines: Hammersmith and City, District, Circle and Bakerloo.
KING's CROSS TUBE - RUSSELL SQUARE
The explosion happens at 0856BST on the Piccadilly Underground line heading north from Russell Square to King's Cross.
Police later confirm at least 21 people have died.
An eyewitness who was on the train says: "I was in the second carriage and I think the explosion was in the carriage in front of me, or maybe was even on the track. The screaming from the front carriage was terrible."
Another passenger says that - after 30 minutes - people smashed windows to get out of their carriage and started walking down the side of the train.
Two hours later, police say a rescue operation is still in place to evacuate some passengers still trapped underground.
King's Cross is a major underground and railway interchange with both commuter and long distance railway lines terminating there.
TAVISTOCK SQUARE / WOBURN PLACE
The explosion at 0947BST rips the top deck off a double decker bus in Woburn Place / Tavistock Square.
The location is near to Russell Square - scene of one of the underground train blasts - and also Euston mainline railway station.
Describing the blast, eyewitness Belinda Seabrook said: "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.
"It was a massive explosion and there were papers and half a bus flying
through the air."
It is unclear how many casualties have resulted from the blast.