Theresa May has announced the security services are raising the terror threat level from "severe" to "critical" in the wake of the Parsons Green terror attack.
The Prime Minister announced the first stage of Operation Temperer would be going into effect, which means armed troops will be sent to key locations including nuclear sites, freeing up armed officers to police the streets.
The announcement followed after a bomb was left on an Underground train on Friday morning, injuring 29.
"Critical" is the highest threat level possible on the terror rating system devised by the Joint Terrorism Analysis Centre (JTAC) and means an attack is considered imminent.
In a televised statement, Ms May said: "For this period, military personnel will replace police officers on guard duties at certain protected sites that are not accessible to the public.
"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."
It was last put into effect following the Manchester Arena bombing which killed 22 people in May.
Scotland Yard's Assistant Commissioner for Counter-Terrorism Mark Rowley said in a televised statement later on Friday evening that they would deploy an extra 1,000 armed officers on the streets of London.
He said the public should expect to see additional police, additional stop and searches and additional road checks in the coming days.
Witnesses reported seing an explosion and a "fireball" on a District Line train at 8:20am.
At least 29 people have been treated in hospital for flash burns and others were injured in the stampede to escape the above ground station. NHS England said none of the victims are in a serious condition.
Footage from the incident showed a burning plastic container in a shopping bag with black, white and red wires coming out of it.
The bomb was reportedly on a timer which could be detonated remotely, security sources have said.
Isis have claimed responsibility for the attack through their propaganda arm, Amaq.
