Animal Justice Project (AJP) activists on the roof of the Darlington Farmers Auction Mart where the NBA – Beef Expo was taking place on Saturday. Photograph: Animal Justice Project Facebook Page

Animal rights activists claim to have been “sprayed with manure” as they staged a demonstration on the roof of a building hosting one of Britain’s largest beef industry events.

Animal Justice Project (AJP) activists clad in black masks and clothing scaled an auction building in Durham where the National Beef Association was conducting its annual Beef Expo on Saturday, and displayed banners opposing animal farming while letting off coloured smoke flares.

A group spokesperson told UK media they had been “covered in excrement” after one event attendee allegedly used a sprayer to blast manure at them.

Reports said another activist was injured when one of their group fell and dislocated a finger after a farmer allegedly ploughed towards a group of protesters in a JCB digger.

The AJP said the aimed to highlight “farmed animal suffering and environmental safety concerns”.

The National Beef Association (NBA) described the event as a celebration of the best of British beef, drawing at least 5000 guests.

Mark Dent, the chair of the Durham Farmers Auction Market, where the expo was held, told the PA news agency he believed the protesters were the sources of “intimidation”.

“We respect people’s right to protest, but the way they go about it doesn’t help their cause.

“There is no respect for people’s property or livelihood. I’m afraid then you lose your moral high ground… It’s a tin roof and they’re jumping up and down on it, and it’s bending.

“They’ve got their faces covered. It’s intimidation (and) threatening behaviour. I’m all for people protesting what they believe in, but it’s the way they go about it – the face coverings, the intimidation.”

“If you have a pair of eyes you will see how important agriculture is around the world at the moment. Food is top of the agenda.”

An AJP spokesperson told the news agency: “We’ve been sprayed with cow poo by one farmer using a machine.

“It has been a peaceful protest and a silent protest and we are overwhelmed and outnumbered by hundreds of angry farmers.”

Durham Police said five protestors came down from the roof of the auction mart at 5pm, having been there since 4am, and all were arrested on suspicion of criminal damage.