Bryony Esther, 32, from Leigh-on-Sea, was on a C2C service when 15-month-old baby Saffron awoke and needed feeding.
Despite "hovering" near the priority seating area no-one offered to move and instead began sniggering, she says.
Her Facebook post about what happened went viral, the Daily Mail said. C2C urged customers to be considerate.
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Speaking to the BBC, Ms Esther said the incident unfolded as soon as she boarded the train with two of her children.
"I sat my five-year-old down on what was an apparently vacant seat quite near the doors and immediately this man told me 'this seat is taken'," she said.
"So we moved and we ended up stood by the toilets for the rest of the journey with a bike shoved into my legs.
"It was quite unpleasant. My baby woke up and cried and she needed nursing back to sleep.
"I hovered by the priority seat hoping to be offered one. I tried to make eye contact with the people sat in the priority seats but they just blanked me.
"One of the people sat in the priority seats was the cyclist."
Ms Esther, who was travelling to Great Ormond Street Hospital to visit her two-year-old son as he recovered from surgery, said she did not feel able to challenge the group.
"I know a lot of people have unseen disabilities so I did not feel able to ask whether they could let me sit down," she added.
"I lifted my top to feed her and then I looked up and realised there were five men watching and sniggering.
"I felt quite intimidated and uncomfortable. I was really upset and I think this is part of wider society.
"This isn't about breastfeeding. This is about a mum having to hold her baby on a moving train."
A spokeswoman for train operator C2C said: "We welcome breastfeeding on all C2C trains and would hope our customers would be considerate to their fellow travellers and give up their seat when requested.
"Unfortunately this does not appear to have happened on this occasion, and we are sorry for any inconvenience this caused."
A woman forced to breastfeed her baby while standing up on a packed train says she was left feeling "intimidated and uncomfortable".