The BBC's Chris Coneybeer reports from the scene of the protest

Thames Valley Police condemned about 250 demonstrators whom officers say showed "complete contempt" for their own and motorists' safety.

The group gathered at the eight-mile A34 road in what had been billed as a peaceful reunion to mark the third anniversary of the start of construction.

But groups of people walked onto the road, which opened two months ago after vigorous environmental protests, and began to stop traffic.

Diversions: Police closed bypass

After some angry exchanges between protesters and motorists, Thames Valley Police closed the road as a number of people refused to leave the carriageway.

During the two-hour protest, some demonstrators threw bricks, stones and manhole covers onto the road, causing what police described as "extensive damage".

A spokesman for Thames Valley Police said one person was arrested at the scene. Demonstrators have moved on to a rally and candlelit vigil at nearby Donnington Castle.

Focus for struggle

The Newbury bypass became the focus for the anti-roads movement in the UK when thousands occupied woodland earmarked for destruction.

Battles: Heavy security needed in 1996

The project took 34 months to complete as protests, involving many environmental groups, attempted to wreck the construction timetable.

At one point hundreds of people set up 29 woodland camps along the route while others chained themselves into treehouses and locked themselves in tunnels.

Hire of security guards for the construction site added an additional £34m to the bill while Thames Valley Police's operation cost the taxpayer £2.6m.

Eventually hundreds of security guards, specialist climbers and cherry-picker cranes were brought in to pluck determined demonstrators from trees.

About 50,000 vehicles passed through Newbury everyday before the bypass opened.