Late Monday, police arrested activists and appeared to use a crowd-dispelling sonic device at the Line 3 pipeline in Minnesota, which would carry oil across sensitive waterways and tribal lands.

The protesters who gathered in the boreal forests of Northern Minnesota came from across the country — Native American tribes and their supporters, environmentalists and religious leaders — all to fight an expansion of Line 3, a $9 billion pipeline operated by the Canadian company, Enbridge, that would carry hundreds of thousands of barrels of oil through Minnesota’s delicate watersheds and tribal lands.

Some said they had come ready to risk arrest by lying down in the path of construction. Others said they were here to support tribes that have been battling oil and gas pipelines for years, including the highly contentious Dakota Access Pipeline.

Late on Monday, police began making arrests after dozens of activists used an old fishing boat, bamboo and steel cable to blockade the road to a construction site off Highway 71 north of Park Rapids. Several hundred others scaled the fence at the site and occupied it, some climbing atop diggers and transformer boxes or chaining themselves to construction equipment.

Police in riot gear “broke through the steel fences and they just began arresting everyone,” said Tara Houska, a tribal attorney and member of the Couchiching First Nation Anishinaabe along the Canadian border. “This is an act of violence on tribal land,” she said.