Line 3, which entered service in 1968, ships crude from the Canadian province of Alberta to U.S. Midwest refiners, and carries less oil than it was designed for due to age and corrosion. Owner Enbridge is replacing the pipeline so it can roughly double the amount of crude it transports. The pipeline is key to landlocked Alberta’s efforts to boost crude exports from its oil sands.

Pipeline opponents say they stand in solidarity with indigenous communities concerned about the risks the line poses to their rivers and lakes.

“For years, we have tried to assert our sovereignty and speak out against Line 3. We still have time to save our sacred waters and land – our life sources,” said Dawn Goodwin, co-founder of the RISE Coalition, which opposes the project.

Organizers of the protests in Hubbard County said the demonstrations were the largest to date against Line 3. The clashes came after a peaceful march early on Monday that drew hundreds of people.

Between 50 and 100 demonstrators occupied the pump station and barricaded its entrances, chanting “Keep it in the ground” and “Stop Line 3”. At one point a police helicopter hovered low over activists, sending dust and debris flying.

Calgary-based Enbridge and opponents are waiting for a ruling from the Minnesota Court of Appeals, due by June 21, on whether there is sufficient evidence of crude oil demand to justify replacing Line 3.

“We respect everyone’s right to peacefully and lawfully protest, but trespass, intimidation, and destruction are unacceptable,” Enbridge said in a statement.

The company said the protests had so far had relatively little impact on construction and Line 3 is on schedule to be in service by the fourth quarter.