The "family of travelers" said they moved into the 2.5 acre site, which they purchased last year, on Easter Sunday to avoid interference from law enforcement officers while they were on vacation, The Daily Mail reported Monday.
The group spent 48 hours erecting electricity cables, water supplies and bases for mobile homes -- one for each of the 16 families in the group.
"This has been a big rush job and we purposely chose the Easter holiday weekend so that we could get more done before anybody tried to stop us," said Tom Brazil, 30, a father of three.
"There are 16 families here, each with up to three caravans, and we banded together to buy the land 12 months ago," he said.
The site is only 200 yards from the country home of Olympics Minister Tessa Jowell.
"They have done a hell of a lot in a short time. I am genuinely sympathetic to people like them," said Jowell's estranged husband, David Mills, 53, who lives permanently in the country home. "We all need a place to live but, equally so, all have to obey the planning laws."