Trump blasts 'loser terrorists' behind London train attack

President Donald Trump has blasted the "loser terrorists" behind an attack on a London Underground train that has injured at least 18 people.

"Another attack in London by a loser terrorist," Mr Trump wrote on Twitter.

"These are sick and demented people who were in the sights of Scotland Yard. Must be proactive!"

He then added: "Loser terrorists must be dealt with in a much tougher manner," without giving details of what that meant.

"The internet is their main recruitment tool which we must cut off & use better!" he added, also giving no details.

British Prime Minister Theresa May said her "thoughts are with those injured".

She will chair an emergency cabinet meeting in the coming hours.

"My thoughts are with those injured at Parsons Green and the emergency services who, once again, are responding swiftly and bravely to a suspected terrorist incident," Mrs May said.

At least 18 people have been taken to hospitals across London after being injured after a blast sent a "fireball" down a train carriage at Parsons Green station about 5.30pm AEST.

A massive manhunt is unfolding across London as police race to catch the person or people responsible for the attack.

Witnesses have described seeing bodies piling up and people “covered in blood” after seeing a “flash” before hearing a bang.