The lawsuit filed Monday by Earthjustice on behalf of several groups challenges the reduction of Grand Staircase-Escalante National Monument, which has been a source of Republican frustration since it was created by Bill Clinton in 1996.
The monument near the Arizona border contains a rich cache of dinosaur fossils and vast coal reserves. Trump said in Salt Lake City that Grand Staircase-Escalante National Monument will be reduced by half, from nearly 3,000 square miles (7,770 square kilometers) to 1,569 square miles (4,064 square kilometers).
He also reduced Bears Ears National Monument in Utah. A coalition of American Indian tribes plans to sue over that decision.
Native American leaders say President Donald Trump's move to shrink Utah's Bears Ears National Monument is another insult, one they're vowing to fight in court.
Navajo Nation Vice President Jonathan Nez noted Monday that Trump's move comes a week after he referred to a Democratic senator he doesn't like as "Pocahontas." He says the administration doesn't respect indigenous people.
Trump said in a speech Monday that many native people support the move. Nez called that event "a circus."
Representatives from the five tribes who advocated for creation of the monument said Trump decided to reduce it without meeting with them, despite multiple requests.
Zuni councilman Carleton Bowekaty called the move an illegal and shameful attack on tribes.
Shaun Chapoose with the Ute Tribe says Trump's decision benefits "a few powerful Utah politicians."
The Republican chairman of House Natural Resources Committee is applauding President Donald Trump for reversing what he called "abuses" of the Antiquities Act by former Democratic presidents.
Rep. Rob Bishop of Utah says proclamations that Trump signed Monday shrinking Bears Ears and Grand Staircase-Escalante national monuments "are a first step toward protecting identified antiquities without disenfranchising the local people who work and manage these areas.
House Democratic Leader Nancy Pelosi, Senate Democratic Leader Chuck Schumer and other Democrats says Trump is undermining "the preservation of some of this country's most important national monuments" by removing protections for more than 2 million acres (3,125 square miles) of public lands.
The lawmakers say the lands "will now be put at risk of desecration and looting."
Demonstrators are blocking traffic on a major Salt Lake City street during a visit by President Donald Trump as they protest reductions to two national monuments in Utah.
The largely peaceful crowd that lined up near the state Capitol for hours in the snow Monday morning started marching downtown as Trump finished announcing the cuts to the Bears Ears and Grand Staircase-Escalante national monuments.
The crowd of about 3,000 protesters carried signs saying "Protect Utah's Wild Lands" and chanting "Lock him up." They took to the streets after some took a knee, thumped their chests in unison and turned their backs.
Riot police earlier cleared protesters from The Church of Jesus Christ of Latter-day Saints' Welfare Square.
Meanwhile, Mayor Jackie Biskupski tweeted a list of things Trump should target instead, including carbon emissions.
President Donald Trump has signed proclamations to formally scale back two sprawling national monuments in Utah: Bears Ears and Grand-Staircase Escalante.
Trump traveled to Salt Lake City to make the announcement and sign the declarations. The move is supported by Utah's top Republican officials but opposed by tribes and environmental groups.
Trump said in a speech Monday at the Utah State Capitol that past presidential administrations had "severely abused" the purpose and spirt of a federal law that allows them to protect public lands by turning them into national monuments.
Trump says his action means that "public lands will once again be for public use."
Roughly 2,000 demonstrators are lined up near the Utah State Capitol protesting President Donald Trump's announcement reducing two national monuments in Utah.
The vocal group gathered in wintry weather Monday morning held signs like "Keep your tiny hands off our public lands" and chanted "Lock him up."
Navajo protester and University of Utah student Shaniah Chee says wood-gathering in the area is essential to her tribe's traditions. She calls the reduction a major loss.
A handful of counter-protesters also gathered to support Trump and the decision to scale back the Bears Ears and Grand Staircase-Escalate national monuments. Jason Lutu of Lehi works in construction and says he's in favor of potential drilling or mining that could create jobs.
President Donald Trump says he's encouraging 83-year-old Republican Sen. Orrin Hatch of Utah to run for re-election.
Hatch joined Trump on a visit Monday to Salt Lake City. When Trump was asked if he wants Hatch to run for an eighth term in 2018, he replied, "Yes."
Trump made the comments while touring a food distribution center run by The Church of Jesus Christ of Latter-day Saints.
Hatch has not publicly declared his plans.
President Donald Trump is meeting with Mormon leaders during a trip to Utah to announce he's scaling back two sprawling national monuments.
Trump on Monday visited Welfare Square, a complex in Salt Lake City run by The Church of Jesus Christ of Latter-day Saints that provides aid to the poor.
Trump spoke briefly and publicly praised the church for the way it takes care of people.
The Republican president traveled west to announce his intention to shrink the Bears Ears and the Grand Staircase-Escalante national monuments spanning millions of acres in Utah. He has condemned the act of creating the Utah monuments as a "massive federal land grab."
The move is welcomed by the state's top Republicans but opposed by Native American tribes and environmental groups.
President Donald Trump has arrived in Utah to announce that he is scaling back two sprawling national monuments in a move welcomed by the state's top Republicans but opposed by Native American tribes and environmental groups.
Trump landed in Salt Lake City late Monday morning. He was accompanied on Air Force One by Utah Republican senators Orrin Hatch and Mike Lee, and Interior Secretary Ryan Zinke.
The president traveled west to announce his intention to shrink the Bears Ears and the Grand Staircase-Escalante national monuments spanning millions of acres in Utah. The two monuments were among 27 that Trump ordered Zinke to review earlier this year.
Trump will also meet with leaders of The Church of Jesus Christ of Latter-Day Saints while he is in Utah.
President Donald Trump says his plan to scale back two national monuments in Utah is an important move for "state's rights" as well as for the people of Utah.
Trump commented Monday as he left the White House for a trip to Salt Lake City, where he was outlining his intention to shrink the Bears Ears and the Grand Staircase-Escalante national monuments. Both monuments span millions of acres in Utah and are among 27 national monuments that Trump ordered his Interior Secretary to review earlier this year.
Trump previously had condemned the act of creating the Utah monuments as a "massive federal land grab."
Utah Republican leaders had complained that the monuments locked up too much federal land.
Both monuments were created by Democratic Presidents Barack Obama and Bill Clinton.
President Donald Trump is outlining his plans to scale back two sprawling national monuments in Utah, responding to what he has condemned as a "massive federal land grab" by the government.
Trump is traveling to Salt Lake City on Monday to announce his intention to shrink the Bears Ears and the Grand Staircase-Escalante national monuments spanning millions of acres in Utah. The two national monuments are among 27 that Trump ordered Interior Secretary Ryan Zinke to review earlier this year.
Utah's Republican leaders pressed Trump to launch the review, saying the monuments declared by Presidents Barack Obama and Bill Clinton locked up too much federal land.
The planned changes have angered tribes and environmentalist groups who have vowed to sue to preserve the monuments.