A homemade bomb exploded on a packed London subway train during the Friday-morning rush hour, unleashing panic and injuring at least 29 people in a terror attack that was claimed by ISIS.

The blast sent a fireball through the London Underground train at the Parsons Green station at around 8:20 a.m. and sparked a massive manhunt for the terrorist who detonated the crude device, which experts say only partially exploded.

It was fifth terror attack in Britain this year alone, and Prime Minister Theresa May raised the nation’s terrorism threat level from “severe” to “critical,” warning that another strike could be imminent.

“Clearly, this was a device that was intended to cause significant harm,” May said, calling it a “cowardly attack.”

The improvised explosive device was stuffed in a white bucket encased in an insulated bag. Officials suspected it went off prematurely.

While no arrests had been made as of Friday evening, UK authorities had identified a person they believed was responsible, NBC News reported.

The blast sent 29 people, including a young boy, to the hospital — some with flash burns and others with injuries sustained during the ensuing stampede of panicked subway riders.

No one suffered life-threatening injuries, and at least eight people had been discharged by Friday evening.

One subway rider recalled seeing flames rushing through the train.

“I looked around, and this wall of fire was just coming toward us, so we just ran,” commuter Lauren Hubbard said.

“The fireball came at us. If the doors were shut, people would have been engulfed.”

People on the train, which was at a station in an affluent part of Southwest London, screamed and ran for their lives.

“I was on second carriage from the back. I just heard a kind of ‘whoosh.’ I looked up and saw the whole carriage engulfed in flames making its way towards me,” Ola Fayankinnu told Reuters.

“There were phones, hats, bags all over the place, and when I looked back, I saw a bag with flames.”

Witnesses described pandemonium as bloodied passengers fled across a crowded platform, pushing toward exit stairs.

“I was on the Tube when suddenly there was a bang to my left, and it looked like something burning was flying towards me,” Carina Heidrich told The Guardian

“It seemed to come from three carriages down from where I was. Then panic erupted and everyone just tried to get out of the tube. There were screams everywhere.”

People recalled getting crushed and pushed on stairs in the midst of the chaos, as parents cried out for their children.

“I ended up squashed on the staircase. People were falling over, people fainting, crying. There were little kids clinging onto the back of me,” Ryan Barnett said.

Homes in the immediate area were evacuated following the blast, and the Lady Margaret School, an all-girls secondary school less than 330 feet from the subway station, was put on lockdown for several hours.

“I was right outside the corner shop when people started running,” said student Emmanuella Mensah, 16. “People were shouting, ‘Run, run!’ I saw old people, people with their kids. Then someone shouted, ‘Terrorist!’ ”

“There were people sitting on the pavement crying and in hysterics,” she added. “Schoolgirls were coming from all kinds of directions.”

Assistant Commissioner Mark Rowley, the Metropolitan Police’s counterterrorism chief, said late Friday that police had made “excellent progress” in their search for the attacker.

“Hundreds of police officers are pursuing numerous lines of enquiry, trawling through hours of CCTV footage and speaking to witnesses,” he said.

“The device and remnants left at the scene have been taken away and are being examined by forensic experts. While this work is ongoing, there is no doubt in my mind that those responsible intended to cause great harm and injury.”

ISIS claimed responsibility for the subway bomb hours after the incident, announcing that a “detachment” was behind the attack.

“The bombing of the IED in the London tube was carried out by a unit affiliated to the Islamic State,” the group’s official Amaq news agency said, according to The Independent.

Shiraz Maher, an expert on radicalization at Kings College London, told The Independent it was unusual for ISIS to claim responsibility while an attacker was still at large.

According to the SITE Intelligence Group, the term “detachment” is typically used to describe an ISIS attacker or “soldier.”

Explosives experts said the bloodshed could have been far worse if the device had fully detonated.

“There was a bang, a bit of a flash, and that would suggest that, potentially, some of the explosive detonated, the detonator detonated, but much of the explosive was effectively inert,” said Chris Hunter, a former British army bomb expert.

The blast was the fifth terror attack to strike Britain this year — and the fourth in London.

The London Underground, also called the Tube, has been targeted by terrorist several times, including in July 2005, when suicide bombers killed 52 people by blowing themselves on three subway trains and a bus.

London Mayor Sadiq Khan condemned the violence in a Facebook post Friday, urging residents to “remain calm and vigilant.”

“Our city utterly condemns the hideous individuals who attempt to use terror to harm us and destroy our way of life,” he wrote.

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