A second man has been arrested by detectives from the Met's Counter Terrorism Command investigating the terrorist attack at Parsons Green on Friday, 15 September.
He was arrested under section 41 of the Terrorism Act and taken to a south London police station where he remains in custody.
Earlier on Saturday, the police had arrested an 18-year-old man in Dover in connection with the attack.
Regarding the first arrest, Metropolitan Police Deputy Assistant Commissioner Neil Basu said in a statement. "We have made a significant arrest in our investigation this morning. He was arrested on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism. He has now been transferred to custody at a police station in London."
Regarding the investigation, police have said that they've "identified 121 witnesses so far, and we have spoken to 100 of them already. Officers continue to trawl through many hours of closed circuit television footage and more than 180 videos and pictures that have been sent to them by the public. We continue to take large numbers of calls from the public via the confidential Anti Terrorist Hotline."
Prime Minister May has also announced that the terrorism threat level had been raised to critical.
30 people have received treatment at hospital for injuries following the attack at Parsons Green.
The Met Police are encouraging anyone with footage or images from the incident at Parsons Green is asked to upload them at www.ukpoliceimageappeal.co.uk where they will be looked at by investigators.
Anyone with information is urged to call the Anti-Terrorist Hotline on 0800 789 321 or, in an emergency, always call 999.
