Updated at 9:20 a.m. ET
London's Metropolitan Police are investigating an explosion that struck a subway station Friday morning and injured at least 23 people, calling it a terrorism incident.
"We can confirm that we treated a total of 19 patients — mostly for minor injuries — and took them to three London hospitals" after the incident at the Parsons Green metro station," said Natasha Wills of the London Ambulance Service.
Four others went to London hospitals on their own after the blast at the above-ground station, the National Health Service says.
No deaths have been reported as a result of the blast, which seems to have left a small fire burning in a bucket inside a train car. Emergency calls went out at 8:20 a.m. local time, when a fire was reported at the station.
"There was an explosion on a tube train," Assistant Commissioner Mark Rowley said at a news briefing. "We now assess that this was a detonation of an improvised explosive device. "
Of those who were injured, Rowley said, "I understand most of those to be flash burns."
Photos of some of the people who were in the area of the blast show singed hair and skin reddened by intense heat.
When Rowley was asked if a suspect is in custody — or whether it's known if the person responsible for the device was still on the train when it exploded — he stated that the investigation was ongoing, and that hundreds of detectives are working on it, reviewing .
NPR's Frank Langfitt reports for our Newscast Unit:
"Photos posted on social media show a white plastic bucket with flames coming out of the top — along with wires. The bucket is sitting next to a subway train door. A purse, apparently abandoned by a fleeing passenger, sits in the foreground.
"The explosion occurred at the Parson's Green station in Fullham, which is about four miles southwest of Big Ben."
Witnesses say a panic broke out after a fiery explosion hit the train car; hundreds of people rushed to get out of the station. Police are asking members of the public to share any images and information they may have that are related to the scene.
"The Metropolitan Police have confirmed that the explosion on a train at Parsons Green Station this morning is being treated as terrorism," London Mayor Sadiq Khan said. "Our city utterly condemns the hideous individuals who attempt to use terror to harm us and destroy our way of life."
Khan added that he's attending an emergency COBRA security meeting with British Prime Minister Theresa May this afternoon.
President Trump said via a tweet, "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 United Kingdom has suffered four terror attacks since March, including vehicle attacks on London and Westminster Bridges and a bombing outside a concert in Manchester.
Providing more details of this morning's incident, Metro.co.uk reports:
"A bomb squad and armed police have all been seen arriving at the station following the blast which happened in a white bucket inside a Lidl carrier bag.
"Wires could be seen coming out of the bucket which was on fire.
"A metro.co.uk reporter at the scene described seeing people with facial burns, adding that they were 'really badly burned' and 'their hair was coming off'.
"Fire crews and paramedics also rushed to the scene to help people after the fireball went down the carriage.
"They said that the rear of the train filled with smoke and people left the train, some panicking, at Parsons Green."
The incident happened during rush hour when the Underground system is crowded. Passengers were advised to use alternate routes.