The Anti-Harassment Club said it had received about 100 reports on its website since it launched in late-June.
The group said too many young women and girls had become used to facing harassment on the street and were reluctant to contact police.
Brighton and Hove City Council said it took the issue “very seriously”.
One young woman wrote on the website that she had been revising on Brighton Beach when a man sat next to her and “began stroking my hair, legs, and then proceeded to pull my dress up to see my knickers”.
Another wrote a stranger approached her and “put his hand on my upper leg and tried to move it up my skirt” as she sat on the beach.
All reports on the website are anonymous.
Sussex Police said it is “always supportive of crime prevention awareness”, but urged anyone who believes they may be a victim of a sexual assault or harassment to report it to them. Image caption Sussex Police asked victims to report any crimes to them
Co-founder of the group Emma Wilson said the overwhelming response had led them to launch a campaign to make gender-based street harassment, like catcalling, a specific criminal offence.
“As we started to see more and more stories come in, we decided it was about time somebody really did something about it,” she said.
“That sexual objectification in itself does leave you with a shock and a trauma feeling,” Ms Wilson added.
“It does leave you feeling like you have been violated.”
Councillor Steph Powell said the local authority would work with the group to “explore anything that could help improve support,” adding: “Gender-based harassment, violence and aggressions are not acceptable.”