Protesters confront Irish police over a planned Shell pipeline in County Mayo

A group of around 15 masked men, carrying bars, chains and clubs vandalized a controversial Shell pipeline site in County Mayo, Ireland, police said Thursday.

This attack is the latest chapter in a four-year dispute between the major multinational company, who are in the process of constructing a natural gas pipeline 80 miles off-shore in the Atlantic, and a very determined opposition.

The Irish government has given the multinational permission to start pumping natural gas from the untapped field.

Opponents say that Shell will damage the local environment of this rural, remote area, and argue that Shell should fully refine the natural gas off shore. But Shell says that this would be much more expensive and dangerous. The planned refinery, instead, is going to be five miles inland

Reports say that late on Wednesday night, a group of men forced security guards off the pipeline site. One guard was injured when he was hit with a steel pipe on the wrist, Irish police said. The security guards abandoned the site and called for help.

The police found a sledgehammer, iron bars and fence posts with nails were found at the site, which is now a crime scene.

The gang also hot-wired a construction vehicle at the site, smashing a perimeter fence and a security hut with it.

Local police officer Tony McNamara, told the Irish Times that the incident appeared to be "well planned" and was "an almost military-style operation".

"We would ask people to stand back and take stock of what happened down there last night. People are absolutely entitled to protest but it should be peaceful and within the law," he said.

The dispute has become particularly nasty on a number of occasions:  opponents have gone on hunger strike, and have held sit-down protests near the refinery site, which have an occasion required around 200 police.

In 2005, a group of men known as “The Rossport Five” served over 90 days in jail after they refused to obey court orders to allow Shell access to their lands.