A gang of ten young teenagers broke into a building site at Aycliffe Road in Borehamwood on Saturday evening and started throwing bottles stones at buildings and machinery.
The attack was the latest in a series at the Aycliffe Road bridge, which is being strengthened by building contractors employed by the county council.
A security guard in the compound called the police, but by the time they arrived the youths had fled.
"It has been going on for several weeks," said a council spokeswoman."They damaged a CCTV camera the contractors had put up to protect the site. There has also been damage to passing cars, and windows were broken on the site."
Police confirmed that they attended the scene on Saturday, but could take no further action because the offenders had left.
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