Police probing Friday’s terrorist attack on a London tube train have arrested a man at a UK port.

Officers detained the 18-year-old at Dover on Saturday morning.

They later evacuated an area around a house in Surrey, southern England, as officers searched the property.

‘Significant arrest’

Neil Basu, deputy assistant commissioner at the Metropolitan Police, 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.”

Was anyone killed in the attack?

No. However, at least 29 people were injured when the improvised explosive device was detonated on a tube train at around 0820 on Friday morning.

Massive manhunt underway in London after terrorist bomb attack on subway https://t.co/7IArybkZpP#ParsonsGreenpic.twitter.com/arVDNkc7lM — dwnews (@dwnews) 15 septembre 2017

What have the police said?

That the investigation is progressing quickly and their priority is to identify, locate and arrest those responsible.

“Hundreds of police officers are pursuing numerous lines of inquiry, trawling through hours of CCTV footage and speaking to witnesses,” Assistant Commissioner Mark Rowley told reporters.

“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,” added Rowley, who is also the head of National Counter Terrorism Policing.

“My thoughts are with all those who have been affected by this terrible attack. It is an attack on all of us.”

“I urge communities across the UK to be vigiliant and report any suspicious behaviour to the police immediately. Please do not hesitate, no matter how insignificant you may think the information you have is – please let us decide. It could be crucial.”

Have the police spoken to anyone so far?

Yes. Around 45 witnesses as well as members of the public who contact the confidential anti-terrorist hotline.

77 images and videos have been sent in via the UK Police Image Appeal website

Anyone with images from the scene at #ParsonsGreen tube station is asked to upload them to https://t.co/l9dn1FQr7Bpic.twitter.com/6ZzZBnlNG6 — Metropolitan Police (@metpoliceuk) 15 septembre 2017

What about the threat level?

It has been raised to critical by the independent Joint Terrorism Analysis Centre.

This means experts think that a further terrorist attack is not only highly likely, it may be imminent.

Military personnel have been drafted in to protect national infrastructure sites. This allows additional armed police officers to carry out patrols.

Police say more officers will be visible on patrol on foot and in vehicles over the weekend. In particular, they will be present in crowded places, iconic sites, transport hubs and ports.

The national threat level has been raised to ‘critical’ after the terrorist attack at Parsons Green. My statement: https://t.co/bM5BgufWeZpic.twitter.com/wZB35asRpo — Mayor of London (@MayorofLondon) 15 septembre 2017

What does this mean for the public?

There may be disruption for those who are travelling. Police are asking the public to be patient and plan ahead, allowing extra time for any delays.

Officers are also reviewing security at events. Security checks will be enhanced, which may lead to delays.