An 18-year-old man has been arrested in connection with the Parsons Green Tube bombing that left 30 people injured.

Metropolitan Police Deputy Assistant Commissioner Neil Basu described the arrest in Dover, Kent, as "significant".

But he added: "Although we are pleased with the progress made, this investigation continues and the threat level remains at critical."

The teenager was held on suspicion of a terror offence in the port area of the town, which is the busiest ferry hub in Europe and serves as a commercial gateway to the French coast.

:: Live updates: Parsons Green blast


Image: Police have so far spoken to more than 40 witnesses

Met chief Cressida Dick later told Sky News the force had made "considerable progress" in its investigation into the explosion, which happened during the Friday morning rush hour as the District Line train was pulling into the west London station.

Sky sources said the device, which was inside a bucket concealed within a supermarket carrier bag, had a timer and failed to fully detonate.

It is not yet known whether the bomb went off at its intended target.

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Image: A total of 30 people were injured in the blast during the morning rush hour

Of those injured, 19 victims were taken to hospital by ambulance, while 11 presented themselves for treatment. Three people remain in hospital.

The attack sparked a huge manhunt for a potential network of plotters, with the country's top counter-terrorism officer suggesting more than one suspect could have been involved.

The UK's terror threat level was raised to critical - the highest level - late on Friday, meaning another attack is expected imminently.

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Witness describes panic on Tube in the aftermath of the explosion

Mr Basu said the public should "remain vigilant", and added the extra security measures put in place in the wake of the blast will remain in force.

Metropolitan Police Commissioner Cressida Dick said: "London has not stopped after other terrible attacks and it will not stop after this one."

London mayor Sadiq Khan said: "The police investigation is ongoing and there will still be significant activity today and over the days ahead.

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PM: A further terror attack may be imminent

"I am sure I speak for London when I say we are incredibly grateful to the police and intelligence services for doing everything possible to keep Londoners safe."

Although police have described the arrest as significant, the Met later added they would not be releasing where the suspect is from, whether he is the suspected bomber or the precise details of the offence for which he has been arrested.

Police said 45 witnesses have been spoken to and 77 images and videos have been sent in by members of the public.

The Met has renewed its appeal for information, asking anyone with information to call the anti-terrorist hotline on 0800 789 321 or 999 in an emergency.