A second man has been arrested by counter-terrorism police in connection with the terrorist attack on Parsons Green London Underground train.

Met Police say the 21-year-old was arrested in Hounslow at around 11.50pm last night (Saturday, September 16).

He was arrested under section 41 of the Terrorism Act and taken to a south London police station where he remains in custody.

It follows the arrest of an 18-year-old man was detained at Dover port yesterday morning.

Deputy Assistant Commissioner of the Metropolitan Police, Neil Basu, described the first arrest as “significant”.

He added: “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.”

Prime Minister Theresa May announced last night that Britain is on the highest terror alert of “critical” – meaning another attack is expected imminently.

The heightened security came after an improvised device detonated on a District Line train at Parsons Green station, injuring 30 people, three of whom remained in hospital care this morning. It reportedly contained nails and the explosive triacetone triperoxide (TATP), but is thought to have only partially detonated from inside a bucket.

The Met said 45 witnesses have been spoken to and 77 images and videos sent to detectives by members of the public.

The country’s top counter-terrorism officer suggested last night that more than one suspect could have been involved in the attempt to slaughter commuters in a tube carriage.

Assistant Commissioner Mark Rowley said police were “chasing down suspects”, while suggestions were made by Donald Trump that the bomber was known to Scotland Yard.An armed police operation is also currently underway by the counter-terror officers investigating the bombing.

A house is being searched and residents close to the address in Sunbury-on-Thames, in Surrey, are being evacuated, Scotland Yard said.

The evacuation began at 1.40pm following this morning’s arrest.

Mojgan Jamali, 33, who lives in Cavendish Road, Sunbury-on-Thames, said she was told by police she had “one minute” to leave her home.

The mother-of-three told the Press Association: “I was in my house with my children and there was a knock at the door from the police.

“They told me to leave. They said: ‘You have one minute to get out of the house and get away.’

“I just got out, I got my three children and we left the house and the street. We didn’t know what was going on. There was a lot of rumours going on, a lot of stories, people saying this and that, but we didn’t know.

“Earlier the police came around and asked if we could look after another family in our home for a little while, which I was obviously happy to do.

“But then they came back later and just said we had one minute.”

Scotland Yard said it was a “precautionary measure following the arrest of a man in Dover”, and a 100-metre cordon has been put in place.

Amber Rudd has called the arrest “very significant”, but said the operation was still ongoing.

Police have declined to say where the suspect was from, whether he was the suspected bomber, or the precise details of the offence for which he was arrested.

Three victims of the attack remain in hospital, NHS England said, while Parsons Green station was reopened in the early hours of this morning. #

A renewed appeal for information was put out by the force, asking anyone with information to call the Anti-Terrorist Hotline on 0800 789 321 or 999 in an emergency.