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.
Live: For more on this and other stories from Essex
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."

Louis and Marie-Zelie Guerin Martin, who lived in France in the 19th Century, were the parents of the much venerated Saint Therese of Lisieux.
The couple had nine children, four of whom died in infancy. The remaining five, all girls, became nuns.
The youngest, Therese, died of tuberculosis aged 24 in 1897 and was canonised in 1925.
Known as "the little flower", Therese of Lisieux is widely venerated for the simplicity of her spiritual life.
Her autobiography, The Story of a Soul, has inspired generations of modern Catholics.
Her parents are being held up as a fine example of a Christian family while bishops from around the world are gathered here to discuss how best to minister to modern families - including divorced persons who remarry, who - under present Church rules - are barred from taking communion.
Pope Francis told the 50,000 people attending the ceremony that serving others - not seeking careers - should be the model for today's churchmen.
In his homily, Pope Francis said: "The holy spouses Louis Martin and Marie-Zelie Guerin practised Christian service in the family, creating day by day an environment of faith and love which nurtured the vocations of their daughters, among whom was Saint Therese of the Child Jesus."
The Pope also canonised an Italian priest and a Spanish nun, both of whom spent most of their lives helping the poor.

Inverness Castle Viewpoint allows for 360 degree views of Inverness and surrounding area.
High Life Highland, which runs leisure sites on Highland Council's behalf, said all the free slots were booked by lunchtime on Tuesday.
Entry fees to access to the viewpoint on the North Tower begin on Saturday.
After Easter Monday's opening day, 1,100 slots were available.
High Life Highland said all the slots have been booked.
Highland Council, which owns the tower, said the viewpoint was the first phase in transforming Inverness Castle into a major tourist attraction.
The 175-year-old property is currently the base for the city's courts service, which is to be moved to a new building.
A woman forced to breastfeed her baby while standing up on a packed train says she was left feeling "intimidated and uncomfortable".
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Pope Francis has conducted the first canonisation of a married couple in a ceremony in St Peter's Square.
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All slots giving free access to a new viewpoint on a historic building were booked up less than 24 hours after it was first opened to the public.