London - An explosion on a train at London's Parsons Green underground station was the result of "the detonation of an improvised explosive device" and detectives are now searching for a suspect, police said on Friday.

Metropolitan Police Assistant Commissioner Mark Rowley told reporters that MI5 agents and hundreds of officers were going through CCTV to work out what happened in the incident, which left 22 people needing hospital treatment according to Britain's National Health Service.

It is the fifth terrorist attack in Britain this year and British Prime Minister Theresa May held an emergency "COBRA" meeting of government and security officials to discuss the issue.

May called the blast a "cowardly terrorist attack" which had been intended to cause "significant harm" and said there would be an increased police presence on London underground trains.

Britain's terrorist threat level will remain at "severe" - meaning an attack is highly likely, she added.

The British prime minister also rebuked Donald Trump after the US president tweeted that police had those behind the blast "in their sights."

"I never think it's helpful for someone to speculate on what is an ongoing investigation," she said in a television interview.

"We will never be intimidated or defeated by terrorism"

Earlier in the day, Trump wrote on Twitter: "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!"

London Mayor Sadiq Khan called on Londoners to remain "calm and vigilant" in light of the rush-hour incident in the west of the city.

"Our city utterly condemns the hideous individuals who attempt to use terror to harm us and destroy our way of life," he said in a statement.

"As London has proven again and again, we will never be intimidated or defeated by terrorism."

International condemnation was swift with German Chancellor Angela Merkel telling a news conference that her "thoughts are of course with those injured" and with "the British population."

The London Ambulance Service said in a statement that "none (of the injured) are thought to be in a serious or life-threatening condition."

The station, which is above ground despite being on the underground network, has been cordoned off as experts deal with the device.

Photographs and videos on social media showed a bag containing a white bucket with wires hanging out of it which was partly on fire.

One passenger, who gave his name only as Lucas, told BBC 5 Live Radio: "I heard a really loud explosion - when I looked back there appeared to be a bag.

"I saw people with minor injuries, burns to the face, arms, legs, multiple casualties in that way," he said. "People were helping each other."

London Fire Brigade said that six fire engines, two fire rescue units and around 50 firefighters and specialist officers had been in attendance at Parsons Green.

Transport for London said that services on the District line, which runs through the station, had been suspended and advised passengers to "replan their journeys."

Threat level would remain at "severe"

A Home Office report released on Thursday showed that arrests for terrorism offences made by British police in the 12 months up to June were up 68 per cent to 379 - an annual record.

The total includes dozens of arrests linked to Islamist terrorist attacks this year in London Bridge, Westminster and Manchester.

About one-third of those arrested were charged, including 105 people who were accused of terrorism, while 50 per cent of suspects were released without charge, the Home Office reported.

The Crown Prosecution Service completed 71 terrorism trials, up from 62 in the previous 12 months, it said.

Following the suicide bombing in May that killed 22 people and injured dozens at the end of a concert at Manchester Arena by US singer Ariana Grande, May said Britain's terrorist threat level would remain at "severe" - the second-highest level.

The government has maintained the threat level at "severe" - meaning an attack is "highly likely" - for more than two years.

Rowley recently said that police nationwide handled an average of about 500 active counterterrorism investigations involving 3,000 subjects of interest.

Rowley, Britain's top counterterrorism officer, told the BBC that the police were investigating an "extraordinary diversity" of terrorism cases.

"We've got young people who are being radicalized; we've got men and women; we've got some much older people who are hardened jihadists who've been in and out of prison and want to commit attacks again," he told the broadcaster on Thursday.

"You've got simple attacks with knives and cars, through to the most sophisticated plots."

A map showing the terrorist attacks in Great Britain since 2005.