Britain's terror threat level has been raised from Severe to Critical, meaning another attack may be imminent, in the wake of the Parsons Green train blast.
Extra police and soldiers will be posted around major landmarks and sporting events in London this weekend.
Friday's blast injured 29 people, but it's thought the homemade bucket bomb didn't actually detonate properly: it set off a fireball in the train carriage, but NOT the devastating explosion that was intended.
Some of the injured have flash burns, others were hurt in the crush to get out of Parsons Green station during the busy Friday morning peak.
Islamic State has claimed responsibility for the blast, saying a 'detachment' from its terror group was responsible.
This it Britain's fifth terror attack this year, but it's the only one that hasn't caused fatalities.
"Everyone just piled out of the tube and there was a distinct smell of burning. I certainly saw some burn injuries there as well," one man told the BBC.
"As you can imagine this was the school run time.. a lot of school kids were on that tube," he said.
Another woman's described the crush to escape: "I went down the stairs and after a while people were just piling on top of each other, because people were just trying to run so quickly."