A woman sexually harassed on a tram said the abuse, which she filmed and posted on social media, was "all too common".
Journalist Charlotte Green tweeted a clip of the ordeal on a Metrolink service between Oldham and Manchester's Exchange Square.
It shows a man making lewd comments and sexual gestures before threatening a passenger who tried to intervene.
Ms Green, 26, said the system needed to change.
The harassment took place on Monday at about 20:00 BST on the East Didsbury line as Ms Green made her way home from work. It lasted about 15 minutes.
She said: "This was not rare, it is a problem women face every day on public transport.
"But this crossed a whole new line.
"He got on the stop after me and was high on something... then he turns on the only guy who stepped in and tried to help me."
Ms Green said others on the tram were reluctant to get involved as "these type of incidents have been normalised".
She said: "I don't think the guy who did try and help will ever do it again as he went completely mental on him. It was quite scary.
"We can't rely on the kindness of strangers to step in anyway, what kind of system is that?
"There should be conductors on trams so they could defuse the situation before it escalates and if required discretely alert police.
"Pressing the emergency button and stopping the tram in this instance wouldn't have been a good idea, I think it would have made the situation worse and I just wanted him to get off."
The abuse has been reported to the police and Transport for Greater Manchester.
Danny Vaughan, head of Metrolink, apologised and said the "horrible" harassment was "unacceptable".
He said Metrolink had increased its number of customer service staff and Travelsafe officers regularly patrolled the network to stop "inconsiderate and ignorant behaviour".
Ms Green added: "This shouldn't be my normal commute home from work - or anyone else's - it made me feel objectified, uncomfortable and unsafe.
"A number of my female colleagues have similar or even worse experiences on the tram.
"Since tweeting the video I have received a number of messages from others who have thanked me for raising the issue."
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