British police arrested an 18-year-old man in the southern port of Dover on Saturday in a "significant" development in the hunt for the culprits behind a London commuter train bombing that injured 30 people a day earlier.
Prime Minister Theresa May put Britain on the highest security level of "critical" late on Friday, meaning an attack may be imminent, and deployed soldiers and armed police to secure strategic sites and hunt down the perpetrators.
In the fifth major terrorism attack in Britain this year, the homemade bomb shot flames through a packed commuter train during the Friday morning rush hour in west London but apparently failed to detonate fully.