The "very dangerous" bomb set off on a packed Tube train at Parsons Green station contained a "large quantity of explosive" and was "packed with shrapnel", the Metropolitan Police Commissioner has said.
Cressida Dick confirmed that the improvised explosive had not detonated fully as she said it was lucky no lives had been lost.
"Thank goodness nobody was killed at Parsons Green," she told LBC radio.
"That was a very, very dangerous bomb. It partially detonated. It had a large quantity of explosive and it was packed with shrapnel.
"It could have been so much worse."
Ms Dick also paid tribute to the "wonderful response" of the emergency services attending the incident.
A total of 30 people were injured after the bomb partially went off on the Tube train.
Most of them suffered "flash burn" injuries thought to have been caused when the ignition flame went off - but it is thought to have failed to set alight the main body of explosive material.
Six men have been arrested in connection with the attack, including a boy of 17, and an 18-year-old man believed to be the suspected bomber.
Four men were also held, two of whom were since released from custody without charge.
One of those freed by police was a 21-year-old man who was named by acquaintances as Syrian refugee Yahyah Farroukh.
His mother suffered a heart attack after learning that her son was arrested, said his boss.
She remains in a critical condition in hospital in Egypt, according to Suleman Sarwar, who runs Aladdins Fried Chicken in Hounslow.
He said the takeaway had received "abuse, threats, anger, and hatred" after Mr Farroukh was swooped on by undercover detectives after finishing his shift last Saturday night.
Police continue to question four people, including the 17-year-old boy held in Thornton Heath, south London, and the 18-year-old who was stopped at the Port of Dover.
They have also been given more time to question two men aged 25 and 30, who were both arrested in Newport, south Wales.
Searches at an address in Surrey, one in Thornton Heath and two in Newport are continuing.