On Tuesday night Kaitlyn Regehr, 30, was on the bus home through west London when she says she was sexually assaulted.
"It was 10:30 at night, it wasn't particularly late. I was on the 207 [bus] going through Shepherd's Bush, when a guy copped my bum," she told BuzzFeed News. "I didn't say anything, I just felt uncomfortable and moved out the way.
"I know that sounds ridiculous, [but] I did what most women would do, just move away, and hope that he doesn't get off the bus and follow you.
"I probably wouldn't have done anything more if [another passenger] hadn't saw it. He told the guy who touched me 'what are you doing?
"The guy intervened and it got a little bit heated, but he was clearly saying it loud enough for the bus to hear. He also said 'Do you have any women in your life? Do you have a mother, a sister?', and I was really touched by that, he kind of humanised the situation. But while I was getting off [the bus], I didn't get to thank him properly."
Regehr posted the photo on social media in the hope that someone in her neighbourhood would recognise the man who stood up for her. She said she wanted to say thank you and buy him a pint at her local pub.
Regehr, a PhD student at King's College London, described the reaction to her Facebook post as "amazing". It has been shared over 1,000 times, and people on social media have shown support and shared their own stories of street harassment.
"People have messaged me, not just from London, [but] from all parts of the world. Men have contacted me too. One man contacted me on LinkedIn horrified that someone could feel so entitled to grab a woman's bum."
Although she was initially hesitant, Regehr contacted the police to report the harassment later on Tuesday.
"TFL has a big campaign about that right now, so I texted [them] and they suggested I contact the police, which, at first, I thought was a ridiculous thing to do, but I did. They sent someone over...and there was something quite validating in being taken seriously for it."
I love this story @kaitlynregehr Women aren't objects. https://t.co/lbdUEGRkP3
A wonderful story. As an aside, I would also have stepped in if I saw this happening (and indeed have). https://t.co/pZpO1iZcYd
Regehr said that this is not the first time that she has been harassed by men on public transport, but this is the "first time someone has stepped in".
"I'm sure that it's the same experience for many women. I guess this experience has been a huge eye-opener for how much I let that behaviour go. I felt uncomfortable and kind of vulnerable in that situation, but I probably wouldn't have done anything more if that passenger hadn't stepped in."