Twenty-nine people have been treated in hospitals, mostly for burns, though at least eight have now been discharged.
Police were called at around 8.20am yesteday morning following reports of a fire on the District line train as it pulled in to the station.
Friday's detonation was considered by police to have been partial.
She said: "The public will see more armed police on the transport network and on our streets, providing extra protection".
Islamic State has claimed responsibility for the blast, saying a "detachment" from its terror group was responsible.
"The threat level remains at severe".
He asked the public to remain "vigilant", but said people should "not be alarmed".
Basu said the force was not changing its "protective security measures", adding that steps were still being taken to free up extra armed officers.
Earlier, the British prime minister admonished Trump for his initial reaction to the attack.
"This arrest will lead to more activity from our officer", he said.
The London police counter terrorism command is leading the response and the probe is being supported by the British spy agency MI5, he said.