Supporters of the 400 residents hurled bricks and debris as about 50 riot police officers pushed through makeshift barricades at the site, Dale Farm, near the town of Basildon in Essex County, making at least seven arrests and using an electric stun gun to subdue a protester. The authorities had already cut electricity supplies to the site.

Basildon authorities have been trying to clear the site for a decade, arguing that the 49 plots on the six-acre Dale Farm are in a so-called green belt that is supposed to be kept as open countryside and that is subject to strict zoning laws.

The evictions were the latest chapter of what has become a test case to define the status of the travelers, who have lived across Britain as itinerants and have begun seeking to claim residential rights.

Many of those who have been living in Dale Farm speak with Irish accents and are descendants of itinerants who arrived in England a half century ago and have traveled around Essex County since.

Over time they have become more settled, adopting a more enduring presence in their trailers at Dale Farm. Many residents of the surrounding area have been pressing for the travelers' removal for several years.

This month the authorities won a final legal ruling from a High Court judge that the trailers and their concrete stands could be cleared, while walls, fences and gates could remain. Two days ago, the Court of Appeal rejected a final legal attempt to prevent the eviction.

The move on the site began at first light on Wednesday, witnesses said, when the police officers in riot gear broke down a rear fence, allowing in bailiffs charged with evicting those residents who had not already left.

Supporters of the travelers, some wearing ski masks and hoods, had clambered onto rickety scaffolding, refusing to descend, while others chained themselves together, witnesses said. In one area of the farm, a vehicle had been set on fire.

By midafternoon, the confrontation had settled into an uneasy standoff, with officers trying to pry protesters from the scaffolding.

According to Britain's Press Association news agency, one resident, Nora Egan, said she was struck by a baton as she told the police they were not entitled to break down fences.

Margaret Sheridan, another resident, also claimed she was injured, The Press Association reported. "They're rough, and there is no reasoning with them," she was quoted as saying of the police.

Robert Home, a professor of land management at Anglia Ruskin University, said the travelers would normally be on the road between April and October and use a more permanent site over the winter months. But many had remained at Dale Farm for a longer period to counter the threat of eviction, he said in a telephone interview.

"This is the time of the year when this group would expect to stay put," Professor Home said.

It was not immediately clear where the travelers could go after their eviction, and there was still some resistance among them and their sympathizers to the move, Professor Home said. Three women had chained themselves by the neck to a gate so that they would hang if the gate was forced open, he said.

"It could get quite nasty," he added.