The UK terror threat has been increased to its highest level following the attack on a Tube train in south-west London, the prime minister has said.
Theresa May said the threat had been raised from severe to critical, meaning an attack is expected imminently.
An “improvised explosive device” was detonated at Parsons Green station on a District Line train from Wimbledon.
A hunt is under way for the person who placed the bomb, with so-called Islamic State saying it was behind the attack.
Mrs May said the military would be providing support to police and would replace them on guard duties at certain sites which are not accessible to the public.
She said: “The public will see more armed police on the transport network and on our streets, providing extra protection.
“This is a proportionate and sensible step which will provide extra reassurance and protection while the investigation progresses.”
Twenty-nine people have been treated in hospitals, mostly for burns, following the attack at 08:20 BST.