Anna Gorniack was on her way to work on the District Line when she saw what she describes as a fireball coming her way.
The tube was packed with commuters and schoolchildren when the blast went off at Parsons Green station in west London, which is above-ground.
Speaking of her ordeal, she said: "I'm thankful to God I'm still alive.
"I was on my phone and the next thing I can see is people running and screaming. I looked to my right and I could see a fireball coming our way.
"At that moment I thought 'it's bomb and it's for real' so I started running. Unfortunately I had a fall so I found myself on the floor and at that moment I really felt this is my last moment.
"In my head I was repeating prayers in my head in my native Polish.
"When I saw it was OK to get up and out of the train I did that."
18 people have been taken to hospital - but no one is believed to have life-threatening injuries.
Some have suffered burns - while others were hurt in the panic which followed.
Reflecting on the incident, Gorniak said: "I've never been so close to something life threatening - I just realised how fragile our lives are and we never know when it's our time.
"I wasn't able to be calm and pray. I'm just very thankful that I was protected but I know my life is in someone else's hands.
"I will be praying through the coming days."
A manhunt is now underway for whoever is responsible for planting the device - which appears to have been left in a bucket in a plastic bag.
No one's been arrested.
London's Mayor Sadiq Khan says he's confident police are doing all they can to catch whoever was behind the attack.