A convoy of 74 vehicles blocked the road just north of junction 14 on the border between Gloucestershire and Avon at about 5.15pm. Police wearing riot gear were pelted with stones when they arrived. The southbound section between junction 12, near Hardwicke, and junction 14 was closed and hundreds of motorists were diverted. The northbound carriageway was also closed during the stoning incident but reopened after travellers agreed to meet police negotiators.
The travellers, who had spent the night at a lay-by in Moreton Valence, south of Gloucester, are believed to have felt aggrieved at the extensive police operation, codenamed Nomad, designed to prevent them and other convoys of New Age travellers from holding a holiday festival.
One likely site, Castlemorton Common, Hereford and Worcester, where 20,000 travellers held an illegal gathering last summer, was made subject to a five-mile exclusion order.
A convoy of around 90 vehicles blocked the northbound carriageway of the M5 for two and a half hours on Saturday at junction 19 near Portishead, but there was no violence when police arrived.
In last night's incident, the travellers finally agreed to police demands that they should disperse after talks lasting about 30 minutes. The vehicles were lined up in the slow lane at 6.30pm as police started sending them off in groups of five or six.
Up to 50 officers were involved in the confrontation. A police spokesman said that an unspecified number were held in reserve and that officers from Avon and Somerset, Gloucestershire, Wiltshire and Gwent were involved. There were no injuries or arrests.