The new Adur Ferry Bridge has been professionally cleaned just days after opening because horses have defecated on it.
Tens of thousands of pedestrians have already used the £9.8 million Shoreham footbridge since it opened last week - but it took just four horses to cause the damage.
Adur District Council leader Neil Parkin branded the actions of the people “stupid and dangerous”.
He said: “The chairman of the council had seen two silly people with their horses on the bridge on Saturday.
“They were then writing on Facebook about what they had done. They were bragging about it. But the horses had pooed all over the bridge.
“They were lucky to get lucky away with it. The horses could have bolted.
“The Worthing and Adur Tidy Clean agreed to go down on Sunday and clean it up. Bless 'em.”
He said they were set to return to the bridge yesterday and use high-pressure hoses to wash away the remaining mess.
He added: “It's beyond silly. It is stupid and dangerous. If the horses had seen the river they could have gone in - with the riders.
“Now signage is being put up informing people it's not a bridle path.”
Scores of people also took to social networking sites to complain. One man said: “I love horses, but you have to admit it was a bit off letting them **** on the new bridge on the first week of it being open.”
One of the horse-rider's mothers said she had reported people's comments to Sussex Police.
Stephanie Whibley said on the social networking site she felt her daughter, Holly Whibley, was being abused.
Yesterday she told The Argus: “It was a beautiful day and as a group we thought we would take part in the celebrations.
“It did not say we couldn't ride across the bridge and, as we could the old one, we thought it was ok.
“The majority of people enjoyed the sight and thought it was lovely. Unfortunately, as nature is unstoppable, two of the horses left a deposit.
“The social media is wonderful when used right. However this was a witch hunt.”
An Adur and Worthing Council spokesman, who said she could not estimate how much the clean-up cost, said: “We don't normally cover the bridge as this is the responsibility of West Sussex County Council but we noticed it needed cleaning and on this occasion we gave it a wash."
Below are some tweets posted on the day of the bridge opening, last Thursday: