President Donald Trump speaks at the Utah State Capitol, after he called for a 90 percent reduction in the size of Utah's Bears Ears National Monument and 50 percent reduction to the state's Grand Staircase Escalante National Monument, in Salt Lake City, Utah, U.S., December 4, 2017.

The first proclamation Trump signed will shrink the size of the Bears Ears National Monument by 85 percent, or around 1.1 million acres. The second will reduce the size of the Grand Staircase-Escalante National Monument by another 800,000 acres, to just under half of its current size, according to fact sheets from the White House.

"You know best how to conserve these lands for many, many years to come," the president told the invitation-only crowd.

"I come to Utah to reverse federal overreach and restore this land" to local residents, Trump said in a speech at the Utah state Capitol in Salt Lake City.

President Donald Trump on Monday signed presidential proclamations which vastly reduce the size of two protected red rock canyon areas in Utah: The Bears Ears National Monument and the Grand Staircase-Escalante National Monument.

Rolling back these designations, Trump said, was a way to "give back" public lands to the people who live near them and love them most.

The decision to shrink the national monuments, which were designated by former President Barack Obama, is highly controversial and pits conservationists and recreational users of public lands against the oil and mining industries, as well as other commercial interests.

By drastically reducing the amount of land protected under the Bears Ears and Grand Staircase National Monument designations, Trump effectively opened up the unprotected land to future drilling and mining.

Trump said his proclamations remove "harmful and unnecessary restrictions on hunting, ranching and responsible economic development."

The authority to create national monuments is afforded to presidents under a 1906 law called the Antiquities Act, and in the century since the law was passed, it has been used to protect millions of acres of wilderness and scores of culturally significant sites.

Trump decried what he called "abuses of the Antiquities Act," which he said gave power over land to "far-away bureaucrats."

There are currently more than 125 designated national monuments, encompassing everything from a coral reef in the Caribbean to an African burial ground in New York state to a barrier island off the coast of Alaska.

Trump's decision is expected to spark a protracted legal battle over the president's authority to set aside land for conservation, and the rights of states and industries to access protected land.

Before the president had even finished his remarks on Monday, environmental and conservation groups were already labeling Trump's decision "illegal."

"Trump's unprecedented, illegal action is a brutal blow to our public lands, an affront to Native Americans and a disgrace to the presidency," said Randi Spivak, public lands program director at the nonprofit Center for Biological Diversity.

"He wants to hand over these lands to private industry to mine, frack, bulldoze and clear-cut until there's nothing left for our children and grandchildren," Spivak said in a statement.