An 18-year-old man has been arrested in connection to a terrorist attack on a London underground train, which left 30 people injured when a homemade bomb exploded during peak hour, police say.
The man was in Dover on England's south-east coast on Saturday morning (local time).
Police said the man was being held under the Terrorism Act, and that he was in custody at a local police station and would be transferred to a south London station "in due course".
"We have made a significant arrest in our investigation this morning," Deputy Assistant Commissioner Neil Basu said.
"Although we are pleased with the progress made, this investigation continues and the threat level remains at critical.
"This arrest will lead to more activity from our officers. For strong investigative reasons we will not give any more details on the man we arrested at this stage."
The latest police update also raised the number of people known to be injured from 29 to 30.
A critical threat level means the Government task force believes another attack may be imminent.
The attack at Parsons Green also prompted the Government to take the rare step of deploying soldiers to replace police officers on guard duties at certain protected sites not accessible to the public.
The soldiers will also add to the police presence at public places to deter attacks after the improvised explosive device was partially detonated on a District Line train.
"The public will see more armed police on the transport network and on our streets providing extra protection," Ms May said in a televised statement.
The Parsons Green tube station was reopened on Saturday, restoring some normalcy to London's transport network after a day of severe disruption.
The home-made bomb, which was concealed in a bucket within a shopping bag, went off about 8:20am, as the train was carrying commuters from the suburbs, including many school children.
Officials said the bomb was intended to do grave harm to commuters, and analysts said the injuries would have been far worse had the entire device exploded.
The Islamic State group claimed responsibility for the attack, which it said was carried out by an affiliated unit.
Britain has endured four other attacks this year, which have killed a total of 36 people.
The other attacks in London, near Parliament, on London Bridge and near a mosque in Finsbury Park in north London, used vehicles and knives.
In addition, a suicide bomber struck a packed concert hall in Manchester in northern England, killing 22 people.
That attack in May also briefly caused the threat level to be set at critical.