Several environmental activists suspended themselves from the Fred Hartman Bridge in Houston Thursday morning to protest the use of fossil fuels and challenge Democratic presidential candidates preparing for the debate to hold the industry accountable.

Greenpeace USA, a non-governmental environmental organization, said in a tweet that they were demonstrating at the bridge to "confront" President Donald Trump and "the oil industry."

A video shared by the group shows several protesters using colorful banners to dangle themselves from the bridge over the water.

HAPPENING NOW: We're in the heart of the fossil fuel industry (the largest oil export channel in the US) to confront Trump & the oil industry.



RT to support the brave activists & take action for #GreenNewDeal! >> https://t.co/xGNiM0OlYG pic.twitter.com/xkoyvj2nt6 — Greenpeace USA (@greenpeaceusa) September 12, 2019

Travis Nichols, a spokesperson for Greenpeace, told NBC News that 11 activists are suspended from the bridge and another 11 are there for support. They plan to stay on the bridge for 24 hours, he said.

Harris County Sheriff Ed Gonzalez said the department is monitoring the demonstration, and two northbound lanes are closed.

Assistant Chief Tim Navarre said they don't have plans to remove the protesters because of safety concerns.

"We don't want to cause harm to them at this point," he said at a news conference. "If it becomes a health or safety issue, we may be forced to help them off."

Gonzalez told reporters there are concerns about the heat, which he said is about 90 degrees, and whether the protesters have enough water.

Nichols told NBC the protesters are "hanging in there" and "doing fine, right now."

The demonstration comes hours before the third Democratic 2020 presidential primary debate Thursday night at Texas Southern University.

"We challenge every candidate on stage tonight to promise to hold the fossil fuel industry accountable if they become president," the group wrote, adding the hashtag "PeopleVsOil."