An 18-year-old man has been arrested in connection to a terrorist attack on a London Underground train, which left 30 people injured when a homemade bomb exploded during peak hour, police say.

Key points: Properties evacuated south-west of London as a "precaution" during police searches

Properties evacuated south-west of London as a "precaution" during police searches Police say the arrest of an 18-year-old man is "significant", but investigations are continuing

Police say the arrest of an 18-year-old man is "significant", but investigations are continuing The terror threat level has been raised to its highest point

The man was arrested in Dover, a major ferry port for travel between Britain and France on England's south-east coast, on Saturday morning (local time).

Police said the man was being held under the Terrorism Act and had been taken to London for questioning.

His identity is a closely guarded secret and police have implored the press not to speculate while the inquiry unfolds.

Authorities would not say if they thought the man was trying to flee to France on a Dover ferry.

The fast-moving inquiry shifted to the town of Sunbury in Surrey, south-west of London, where neighbours were ordered to evacuate immediately by police.

Investigators established a 100-metre cordon around a building, police said, adding that the evacuation was a "precautionary measure".

Police raided a small house in Sunbury, Surrey, about 18km from Parsons Green. ( Jonathan Brady via AP )

Mojgan Jamali, who lives near the house being searched, said police gave her "one minute" to pack.

"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'," she said.

Some neighbours were crying and many said they had no idea when they would be able to return to their homes.

Police gave many shelter and food at a nearby sports club.

London doesn't give in: police chief

Police and forensic officers work at a property in Sunbury-on-Thames, in south-west London, as part of the Parsons Green investigation. ( AP: Victoria Jones, PA )

Police did not reveal details about the search, but the precautions suggested concern there might be explosives or violent extremists on the property.

The Islamic State group has claimed one of its units planted the bomb on the train.

Joining officers on the beat, London police chief Cressida Dick said there was a "very considerable threat" in the city and police were "doing everything we can".

"The thing about London is we don't give in," she said.

"We don't give in to terrorists, we never have, and we carry on."

Deputy Assistant Commissioner Neil Basu called Saturday's arrest "significant", adding the investigation was continuing".

"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," he said.

The latest police update also raised the number of people known to be injured from 29 to 30.

Soldiers have been deployed around Britain to aid police. ( AP: Tim Ireland )

Officials have hinted there may be more than one person involved, but have not released details in what is termed an ongoing and covert inquiry.

Police were continuing to comb through CCTV images as part of the investigation.

The train hit by the bomber had video cameras in each car, and the London Underground network has thousands of cameras at the entrances to stations and along the labyrinth of subterranean and aboveground passageways leading from the entryway to the trains.

Britain deployed hundreds of soldiers at strategic sites, including nuclear power plants and defence sites, on Saturday to free up armed police to help in the hunt for those behind the bombing.

Parsons Green station reopened as terror threat level raised

The Parsons Green station reopened on Saturday. ( Reuters: Peter Nicholls )

Authorities increased the terrorism threat level to critical late on Friday, after the bomb partially exploded during the morning rush hour at the Parsons Green station.

A critical threat level means the Government task force believes another attack may be imminent.

Prime Minister Theresa May said raising the threat level to its highest point was a "proportionate and sensible step".

The attack also prompted the Government to take the rare step of deploying soldiers to replace police officers on guard duties at certain protected sites not accessible to the public.

The soldiers will also add to the police presence at public places to deter attacks after the improvised explosive device was partially detonated on a District Line train.

"The public will see more armed police on the transport network and on our streets providing extra protection," Ms May said in a televised statement.

The Parsons Green tube station was reopened on Saturday, restoring some normalcy to London's transport network after a day of severe disruption.

Injuries would have been worse if bomb exploded entirely

Sorry, this video has expired A white container inside a plastic bag burns on the London Underground train. (Photo: Twitter/Rigs)

The homemade bomb, which was concealed in a bucket within a shopping bag, went off about 8:20am, as the train was carrying commuters from the suburbs, including many school children.

Officials said the bomb was intended to do grave harm to commuters, and analysts said the injuries would have been far worse had the entire device exploded.

The Islamic State group claimed responsibility for the attack, which it said was carried out by an affiliated unit.

Britain has endured four other attacks this year, which have killed a total of 36 people.

The other attacks in London, near Parliament, on London Bridge and near a mosque in Finsbury Park in north London, used vehicles and knives.

In addition, a suicide bomber struck a packed concert hall in Manchester in northern England, killing 22 people.

That attack in May also briefly caused the threat level to be set at critical.

Forensic investigators work at the site of the attack at Parsons Green. ( Reuters: Hannah McKay )

AP/Reuters