0850The blast was on Circle line train number 216, which had just left Edgware Road station heading west for Paddington.
Police said that the device was on the floor of the second carriage, close to the first set of double doors.
After the blast passengers began emerging from the underground station, many covered in blood and clearly distraught. Paramedics went inside, prompting fears there may still be injured people trapped underground.
A woman passenger told 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."
Six people died in the attack.