Herefordshire Council wants to build a protective shield over the Rotherwas Ribbon and build a relief road over it.
Dozens of people took part in a campaign outside a council cabinet meeting on Thursday afternoon calling for a halt to road building.
Nine refused to leave the meeting until the subject was discussed.
The item was not on the cabinet's agenda.
'Important as Stonehenge'
Police were called to the council's Brockington building after reports of unrest there.
A West Mercia Police spokesman said: "Eight people have been arrested for aggressive trespass when they refused to leave the council chamber.
"The situation was dealt with very quickly after that. There was no trouble with the people who were outside."
Archaeologists have said the 197ft (60m) long ribbon of stones, found by road builders, could be as important as Stonehenge.
Inspectors from English Heritage visited the site on Monday to investigate if it is worthy to be granted ancient monument status.
If inspectors schedule the monument, work on the road will have to stop.
It is made up of a series of deliberately fire-cracked stones and appears to have been deliberately sculptured to undulate through the whole of its length that has so far been uncovered.
Archaeologists have said there are no parallels to the site in the rest of Europe, with the closest similar artefact being the 2,000-year-old Serpent Mound of the Ohio River valley in the US, a 1,330ft (405m) long effigy of a serpent.
A petition set up on Monday calling for Prime Minister Gordon Brown to step in and save the Ribbon has attracted nearly 500 signatures.
Campaigner Rob Hattersley, who had been taking part in the protest, said: "This shows how strongly people feel about the issue.
"However, although the protest is a good way of putting the topic back in the headlines, we don't want it to overtake the real message we are trying to get out there."
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