Seven campaigners protesting against the route of the M3 motorway near the Hill of Tara in Co Meath were arrested yesterday following clashes with construction workers and security personnel.
One security guard was taken to Navan General Hospital after he received a blow to the face, but his injuries are understood not to be serious.
TaraWatch, the main group protesting against the motorway, has denied any knowledge of the incident.
Five protesters, including NUI Maynooth lecturer Dr Muireann Ní Bhrolcháin, were arrested by gardaí yesterday morning and a further two protesters were arrested in the evening. All appeared at a special sitting of Navan District Court yesterday evening in relation to public order charges.
The protesters were attempting to stop construction workers from entering the Soldier Hill archaeological site on the northern slope of the Hill of Tara at Blundelstown.
Spokesman for the protesters Vincent Salafia said the seven were arrested on the grounds that they were blocking traffic on the road, but he said they were "not in the public highway when protesting".
TaraWatch has always conducted peaceful protests, Mr Salafia said.
He denied that any members assaulted construction workers or security staff.
He claimed campaigners were manhandled by security personnel.
Construction company Eurolink said the protesters' behaviour was becoming increasingly dangerous.
"We have had protesters trespassing on the construction site since construction began in May and they refuse to leave the site and keep away from the heavy construction vehicles which are in use every day," Jose Tamariz, managing director of Eurolink, said.
"We are really concerned that there will be an accident, with protesters and or construction workers injured or killed, as a direct result of the illegal actions of the protesters."
Aggression from the protesters reached its pinnacle yesterday, he claimed.
"We recognise the protesters' right to protest peacefully and legally but we deplore the assault on one of our employees. Workers should be allowed to do their work lawfully, without interference."
Protesters climbed on top of construction vehicles and stood in the way of moving machinery in an effort to prevent the start of work on the site.
Gardaí arrived on the scene at about 9.30am yesterday and remained at the site throughout the day, but were mostly directing traffic through the obstructed stretch of road in adverse weather conditions.
Independent MEP Kathy Sinnott was present at the protest. "Protesters must be within the law, but so must construction workers," she said.
The Love Tara group is to hold a protest in Dublin on Saturday.
It plans to march from the Garden of Remembrance to the Custom House, where it will hand in a petition with 50,000 signatures seeking the cessation of all work on the controversial motorway.