"They don't care for your land like you do," Trump told a crowd in Salt Lake City, the state capital. He said his action would "reduce federal overreach" and "return the rights to this land to your citizens. Public lands will once again be for public use."
The Republican Trump's order undid edicts signed by two former Democratic presidents, Bill Clinton and Barack Obama, that created the 526,000-hectare Bears Ears National Monument and the 769,000-hectare Grand Staircase-Escalante National Monument.
Trump's order would cut the size of Bears Ears by 85 percent and Grand Staircase by half, but must be ratified by Congress. Trump's aim is to open up the monuments to more recreational use and development, including drilling, mining and cattle grazing.
As he left Washington, Trump said, "We're going to be doing something that the state of Utah and others have wanted to be done for many, many years. It will be one of the great, really, events in this country for a long time. So important for states' rights and so important for the people of Utah.'
FILE - In this undated file photo, the Upper Gulch section of the Escalante Canyons within Utah';s Grand Staircase-Escalante National Monument features sheer sandstone walls, broken occasionally by tributary canyons.
But Native American and environmental groups immediately vowed to file legal challenges to block Trump's action, saying the designations of the vast lands as monuments need to remain unchanged in order to protect wildlife, sacred sites and centuries-old artifacts.
The two Utah monuments were among 27 that Trump earlier this year ordered Interior Secretary Ryan Zinke to review. Utah Republican leaders are among those who had urged Trump to launch the review, saying the orders by Clinton and Obama creating the monuments had locked up too much land in the state from development.
In ordering the review of federal control of lands, Trump said it would eventually end 'another egregious abuse of federal power' and 'give that power back to the states and to the people where it belongs.'
The Utah changes are the first time in a half century that a U.S. president has tried to scale back one of his predecessors' protection of vast lands.
But Zinke has recommended that monuments be pared in at least four other states as well.