Female football fans were asked to show their bras to stewards during security searches, a supporters' group has said.
The allegations were made by the Mariners Trust, a Grimsby Town FC supporters club, following a match between the Lincolnshire team and Stevenage FC on Saturday.
In an open letter to Stevenage FC, Mariners Trust Chair Jon Wood, and Alan Rutter, a former football intelligence officer, wrote that "several female supporters were asked to lift their tops up to show their bras to female stewards upon entry to The Lamex Stadium.
"This decision – in the queue in front of other supporters, including men and male stewards – is a gross invasion of privacy.
"Female supporters have also since contacted us to state they were asked by female stewards if they could feel their bras if they confirmed that they were underwired.
"The supporters were made to feel uncomfortable and when they replied they would ‘rather not’ they were ‘reluctantly’ let in to the ground.
"This act would effectively constitute a sexual assault and these types of searches are unlawful."
The letter went on to state that "not all" supporters were subjected to the same search.
Stevenage FC said it was aware of the letter, but would not comment further before an investigation had been carried out.
The club again said it would not comment when asked if asking female supporters to show their bras as part of security checks was a standard search procedure.
The Mariners Trust also alleged that children as young as five were subject to "full body searches", and a number of items which were not on the banned list such as "a tiny bottle of hand sanitiser, contact lens solution, cough sweets and medication" were also confiscated.
The letter also complained that a female steward was posted inside a set of male toilets, there were no washing facilities in the male toilets, and that some stewards at the Lamex Stadium were "antagonistic".
It finished by saying that the issues raised needed to be addressed "as a matter of urgency... to ensure no other visiting team's supporters are treated as animals in the way your club did on Saturday".
Stevenage won the League Two match 3-1.
A spokesperson for Hertfordshire Police said: “Police officers were in attendance at the match but were not involved in any of the searches that were carried out by stewards.
“At this stage no allegations have been made to Hertfordshire Constabulary in relation to the actions of stewards at the football match.
"If any allegations are made to police these will be fully investigated.
“Police were in attendance at the match due to intelligence regarding a previous game between Chesterfield and Grimsby Town, where there was public disorder and members of the public were injured.”