When it exploded, numerous train riders suffered burns, and others were injured as they rushed away from the area of the blast.
London "utterly condemns the ugly individuals who attempt to use terror to harm us and destroy our way of life", Mayor Sadiq Khan said. "For strong investigative reasons we will not give any more details on the man we arrested at this stage", he said.
An improvised explosive device (IED) Friday went off on an underground train during the morning rush hour at a busy station in London, leaving at least 22 people injured, in what Scotland Yard called a "terrorist incident".
Metropolitan Police Commissioner Cressida Dick said: "London has not stopped after other bad attacks and it will not stop after this one".
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Friday's detonation was considered by police to have been partial.
Following the attack, media reported, citing sources, that the police had identified the suspect behind the attack with the help of CCTV camera recordings.
The Manchester attack was the last time that the United Kingdom raised its threat level to critical, its highest level.
The Prime Minister said military personnel would replace police officers "on guard duties at certain protected sites which are not accessible to the public".
Police said they had spoken to 45 witnesses so far and received 77 images and videos from the public, and urged those with information to get in touch.
The Islamic State (ISIS) group has claimed the responsibility for the attack, but Metropolitan Police's Assistant Commissioner Mark Rowley said it was "very routine" for ISIS to claim the attack, whether in contact with those involved or not.