UPDATE 8.30pm: The chief suspect in the Parsons Green bombing was seen being interrogated by two unarmed police officers moments before his arrest in Dover.
Tourist Daniel Vaselicu, 31, had been sitting with a friend in the waiting area at the ferry hub when he saw the man being approached by officers at 7.30am on Saturday.
The departure area was empty apart from the individual, who was said to be young and light-skinned, but not English, the Romanian baker said.
He appeared calm as the officers interrogated him for 10 minutes, leading Mr Vaselicu to believe he was simply homeless.
Moments later, after Mr Vaselicu had left for a coffee and a cigarette, the officers arrested the 18-year-old, who police believe planted the bomb on a packed District Line train on Friday.
A massive counter-terrorism operation then erupted, which saw the ferry terminal evacuated as a precaution.
Mr Vaselicu, who was forced to hang around the area after his car was seized on his way into Britain, told the Press Association: "We were in the seating area at the terminal, the police were interrogating this guy for around 10 minutes.
"It was around 7.30am, he was young, around 25 or under.
"He was white, not Arabic, but he wasn't English - there was only one person there.
"Then we went to the town centre, I wanted to smoke a cigarette and have some coffee, my opinion was that he was a homeless guy and that's why they were interrogating him.
"He was looking normal, not fighting or worried, concerned."
UPDATE 7pm: A teenager suspected of planting the Parsons Green bomb was arrested in the departure area at the port of Dover, Scotland Yard said.
The 18-year-old was detained on suspicion of preparing a terrorist act, sparking an evacuation of the ferry terminal, Deputy Assistant Commissioner Neil Basu said.
Detectives investigating the bombing were said to be "keeping an open mind" about whether more plotters were involved and Mr Basu said the terror threat would remain at its highest level, indicating another attack was feared.
He added: "Officers from Kent Police had to partially evacuate the port of Dover at around 11.40am this morning and this was to enable them to search the premises as a precautionary measure.
"That work is now complete and they have recovered a number of items during that search."
The suspect is now being held at a London police station after being arrested by Kent Police.
Mr Basu said: "At this stage we are keeping an open mind over whether more than one person is responsible for the attack and we are still pursuing numerous lines of inquiry and at great pace."
Echoing the earlier words of Home Secretary Amber Rudd, he added: "All I am prepared to say at the moment is it is a very significant arrest, but as I said we are open-minded.
"If there are other people responsible it's our job to find them and that is part of the reason that we are remaining at critical threat."
EARLIER: A teenager is in custody and a mass evacuation is under way by counter-terror officers investigating the Parsons Green bombing.
Police are searching a house in Sunbury-on-Thames, Surrey, after an 18-year-old - understood to be the suspected bomber - was arrested in Dover earlier on Saturday.
Home Secretary Amber Rudd said it was "much too early to say" whether the bomber was part of the current security picture, following comments from US President Donald Trump that the culprit was known to Scotland Yard.
Speaking after a meeting of the Government's emergency Cobra committee, Ms Rudd said the arrest was "very significant", and added: "We have one arrest and an ongoing operation - when we have more information, we will be sharing it."
It came as residents across Sunbury were forced to flee their homes following the arrest.
Mojgan Jamali, who was among those evacuated from Cavendish Road around 1.40pm, said she was told by police she had "one minute" to pack her bags, grab her children, and leave home as officers carried out urgent operations in the area.
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.
"I only had to open the door and I saw police everywhere."
Mrs Jamali, 33, said she did not know where to go, so decided to take her children - aged between five and 10 - to the shops.
She said: "I had to do something to keep them entertained and to keep their minds off it - it's obviously pretty scary. I'm trying to keep them calm."
Police officers told residents they must wait "at least a few hours" before they could return to their homes, and directed them to the nearby Staines Rugby Club.
Investigators - who it is understood are linking the attack to Islamist extremism - are continuing their probe and the threat level remains at critical, which means a further attack is feared to be imminent.
The country's top counter-terrorism officer indicated that a potential network of plotters could have been involved.
Assistant Commissioner Mark Rowley said police were "chasing down suspects".
Police declined to say where the 18-year-old detained man was from, whether he was the suspected bomber or the precise details of the offence for which he was arrested.
His capture took place in the port area of Dover at around 7.50am, which is the busiest ferry hub in Europe and serves as a commercial gateway to the French coast, including Calais and Dunkirk.
A key strand of the investigation has focused on CCTV as officers comb through footage to establish who planted the device, and when and where it was placed on the train.
Security minister Ben Wallace suggested CCTV images of the bomber could be released as part of the hunt for those responsible, but Scotland Yard subsequently denied there were any plans to do so.
The vast manhunt was launched after an improvised device partially exploded on a District Line train at Parsons Green station during the Friday morning rush hour, injuring 30.
Three victims remain in hospital, NHS England said.
Troops have been dispatched to free up armed police officers after the country's terror threat level was raised to its highest point.
Friday's device reportedly contained the explosive triacetone triperoxide (TATP) and nails, but is thought to have only partially detonated from inside a bucket.
Parsons Green station was reopened in the early hours of Saturday.
There were fears the number of those hurt could have been much higher - with the real potential for life-threatening injuries - had the bomb, which was concealed within a supermarket carrier bag, fully exploded.
It is not yet known whether the device, which was reportedly fixed with a timer, went off at its intended target.
The train - bound for Edgware Road - was just pulling into the station in south-west London when the device detonated in the rear carriage, sending passengers fleeing to safety.