Update (17 September 1pm GMT): A second man has been arrested in connection with the Parsons Green terror attack.
The 21-year-old man was arrested in Hounslow in west London shortly before midnight on Saturday, the BBC reports. He is now being held in custody at a south London police station. A residential property in Sunbury-on-Thames, Surrey is being searched by police in connection with the arrest.
Speaking to Andrew Marr on BBC One this morning, Home Secretary Amber Rudd said the second arrest could suggest the Parsons Green attacker was not a "lone wolf."
She also said there is currently "no evidence" that the so-called Islamic State is responsible for the attack, which left 30 people injured on Friday morning.
"The police are getting on with their operation today. They are working closely with the security services," Rudd told Marr. "They have made two arrests. And they will be working hard to see what else they can find out about this attacker."
NHS England has said that one person injured in the attack is still being treated in hospital.
Update (16 September 11am GMT): An 18-year-old man has been arrested in connection with the Parsons Green terror attack, police have said.
The man was arrested in the Dover port area this morning, The Guardian reports, and is now in custody at a local police station. He is due to be transferred to a south London police station shortly.
Deputy Assistant Commissioner Neil Basu said: "We have made a significant arrest in our investigation this morning. Although we are pleased with the progress made, this investigation continues and the threat level remains at critical.
The public should remain vigilant as our staff, officers and partners continue to work through this complex investigation. We are not, at this time, changing our protective security measures and the steps taken to free up extra armed officers remain in place.
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."
Original story: The UK's terror threat level has been raised to its highest level as the hunt for the perpetrators of the Parsons Green attack continues.
Theresa May announced last night that security services have increased the terror threat level from "severe" to "critical," meaning that "a further attack may be imminent."
The Prime Minister said in a statement that the first phase of Operation Temperer is now being enacted.
"This is a well-established plan to provide military support to the police and for this period military personnel will replace police officers on guard at certain protected sites which are not accessible to the public," she explained. "The public will see more armed police on the transport network and on our streets providing extra protection."
May described this as a "proportionate and sensible step which will provide extra reassurance and protection while the investigation progresses."
The UK's terror threat level was last raised to critical in May in the wake of the Manchester attack.
It is now being reported that a total of 29 people were injured after an "improvised explosive device" was detonated on a District Line tube train at Parsons Green station in south-west London during the Friday morning rush hour.
"Hundreds of police officers are pursuing numerous lines of enquiry, trawling through hours of CCTV footage and speaking to witnesses," Scotland Yard's Assistant Commissioner Mark Rowley told the press last night.
"The device and remnants left at the scene have been taken away and are being examined by forensic experts. While this work is ongoing, there is no doubt in my mind that those responsible intended to cause great harm and injury. So far, detectives have spoken to 45 witnesses and continue to receive information from the public to the confidential anti-terrorist hotline."
Though the so-called Islamic State have claimed responsibility for the attack through its publicity arm, Rowley said it is common for the group to make this move whether or not it was actually actively involved.
Anyone with information about the attack is being urged to call the Anti-Terrorist Hotline on 0800 789 321.