British Prime Minister Theresa May has criticised US president Donald Trump for “unhelpful” speculation about the perpetrators of a train bombing that injured dozens of people.

British Police confirmed they had arrested an 18-year-old man in connection with the attack on Saturday morning (local time), as part of the manhunt to find the terrorists responsible for causing carnage on a south-west London train with a crude homemade ‘bucket bomb’ on Friday.

The teenager was arrested in the coastal town of Dover, about 130kms from London.

Police say 30 people, including a young boy, were injured during the attack on a rush-hour train packed with school children and commuters.

Authorities said they were “chasing down suspects” and trawling through CCTV footage after the bomb partially detonated and set a train carriage on fire at Parsons Green.

In a TV statement on Saturday, Prime Minister Theresa May raised the UK terror threat from ‘severe’ to ‘critical’, prompting fear another attack may be imminent.

Ms May said a major covert investigation was taking place and warned the public of an armed police presence on the transport network and London’s streets.

“This is a proportionate and sensible step which will provide extra reassurance and protection while the investigation progresses,” she said.

The Islamic State group claimed responsibility for the blast, but the London Metropolitan Police Assistant Commissioner Mark Rowley said the announcement was “very routine” for IS.

British Prime Minister Theresa May condemns Trump’s tweets

US President Donald Trump has been condemned for his fast fingers by Theresa May, following the explosion.

Mr Trump was quick to criticise law enforcement ifollowing the attack and implied the assailants had been known to Scotland Yard.

“Another attack in London by a loser terrorist. These are sick and demented people who were in the sights of Scotland Yard. Must be proactive!” the US president said on Twitter.

When asked about Mr Trump’s comments, Ms May said, “I never think it’s helpful for anybody to speculate on what is an ongoing investigation.”

Mr Trump also took the opportunity to highlight “tough talk” on terror such as increasing the travel ban into the United States.

The travel ban into the United States should be far larger, tougher and more specific-but stupidly, that would not be politically correct!

— Donald J. Trump (@realDonaldTrump) September 15, 2017

The quick tweets were a different approach for the US president, who was slow in his response to condemn Neo-Nazis and white supremacists after racial violence broke out in Charlottesville, Virginia, on August 12.

At that time, he said: “When I make a statement, I like to be correct. I want the facts … I don’t want to rush into a statement.”

National Security Adviser HR McMaster later suggested Mr Trump was speaking generally in the series of tweets.

“I think he means generally that this kind of activity is what we are trying to prevent,” Mr McMaster said.

At an event at Joint Base Andrews in Maryland, Mr Trump said he offered prayers for London and said “radical Islamic terrorism … it will be eradicated, believe me”.

The White House said Mr Trump and Ms May spoke on Friday.