Updated: 10:40am  A New Zealander who was just about to get on the tube when the explosion happened is shaken up, but hasn't been put off living in London.

Erika Pinfold usually walks to work, but decided to take the train because she was running late.

She was about to catch a train when she saw crowds of people pouring out of the station in tears.

She said her first reaction first reaction was to call her husband and parents in New Zealand, and then her boss

"I just made my way home and just told my boss I just couldn't come to work because I couldn't face getting on another tube in case [of] something else. You don't know what people are..like if it was terrorist attack if they were planning something else."

She's shocked by what happened but won't be rushing home.

"As much as it would make me want to move home, you can't live your life not experiencing things for something that someone decides to do, because you don't know, you could get killed in a car accident.

Britain's Prime Minister has just raised the country's terror threat level to critical following the London tube attack.

It's the highest possible level and implies that an attack is considered imminent.

Hundreds of police are now involved in a massive manhunt to find the person who placed the bomb.

A total of 29 people have sought hospital treatment, mostly for burns from the flash in the tube carriage but also for crush injuries from the panic to get away.

It's believed an individual has been singled out from CCTV footage, but police are yet to release anything about the identity of the person they're looking for.

The improvised bucket bomb left on a London Underground train bears the hallmarks of previous failed bombings there.

The device was left on a rush hour tube train, and is believed to have been hooked up to a timer.

Most experts think it didn't go off properly and would have been devastating if it had.

22 people have been injured, mostly with burns, from what's described as an improvised bucket bomb.

Photos on social media show flames coming from the bucket, and wires poking out suggesting a timer was used.

Chemistry expert Professor Hans Michels believes the intention was to create a much bigger blast.

"The material that they tried to explode as as a fire and big bomb wasn't of the right composition, the right strength that they wanted to, which means it didn't react quickly enough to deliver the high power shock wave."

London Mayor Saddiq Khan says it's essential people in the city stay calm.

"The terrorists have stepped up attempts to kills us, to injure us, to disrupt our way ofm life and we've got to make sure we're alert, we're vigilant."