Pipeline opponents rappelled down from the Ironworkers Memorial Bridge Tuesday with the intention of stopping an oil tanker from leaving Burrard Inlet.

Greenpeace Canada spokesperson Jesse Firempong told CTV News a dozen people are involved with the blockade, which was launched hours before sunrise.

"They've formed a human drawbridge," Firempong said. "Seven folks have rappelled from the catwalk and they're prepared to stop the tanker from passing."

There is a loaded tanker at Kinder Morgan's Westridge Marine Terminal called the Serene Sea that was due to depart Tuesday, according to Greenpeace.

Kinder Morgan responded to questions about the blockade with a brief statement saying there were “no vessels scheduled to arrive or depart” its terminal in the morning. The company did not answer follow-up questions about whether there are tanker departures scheduled for the afternoon or on Wednesday.

Multiple marine traffic tracking websites list the Serene Sea as being moored east of the bridge.

CTV News spoke with one of the activists involved in the blockade, Will George, as he sat in a climbing harness high above the inlet. He said they are comfortable, equipped with food and water, and in no rush to leave.

"We're going to be here for quite some time, throughout the day and night," he said via video chat.

The dramatic act of resistance is just the latest in a long line of demonstrations against Kinder Morgan and its Trans Mountain pipeline expansion project, which the federal government plans to purchase for $4.5 billion.

After the controversial deal was announced back in May, protesters vowed to ramp up their opposition to the project.

George said he's taking part in Tuesday’s blockade on behalf of his Coast Salish First Nations community.

"I was asked to do this by my elders and my spiritual leaders," he said. "It's in my blood to protect this water."

The seven activists suspended under the bridge are being monitored and supported by five others on the catwalk, Greenpeace said.

A number of large flags are flying from the blockade, which Firempong said were decorated by First Nations from across the Coast Salish region.

Vancouver police told CTV News they started receiving reports of the incident at 7:30 a.m., and that the department's Marine Unit was dispatched to assess the situation from the water. A number of officers were sent to monitor the blockade from land as well.

"Public safety, including the safety of the (protesters) and our officers, is our priority," Const. Jason Doucette said in an email.

There was no traffic disruption on the bridge deck, police added.

With files from CTV Vancouver's Nafeesa Karim and Maria Weisgarber