Environmental groups have filed a lawsuit to block Donald Trump's plan to cut the Grand Staircase-Escalante National Monument almost in half.
The lawsuit was filed by Earthjustice on behalf of several groups suing to block the decision over the nature reserve in Utah.
It comes as five Native American tribes also said they were preparing to take the Trump administration to court over the Bears Ears National Monument in the same state.
The anger follows Donald Trump signing orders to reduce the 1.3 million acre Bears Ears National Monument to 228,784 acres and cut the 1.9 million acre Grand Staircase-Escalante National Monument almost in half in a bid to boost development.
Announcing the move at Utah State Capitol Building in Salt Lake City, Mr Trump said the rollback would "reverse federal overreach" and ensure "public lands will once again be for public use".
The decision has been criticised by several conservation and environmental groups, as well members of the Navajo, Hopi, Pueblo of Zuni, Ute Mountain and Ute Indian tribes, which consider the sites sacred.
The Grand Staircase-Escalante National Monument contains dinosaur fossils and vast coal reserves and the Bears Ears site features thousands of Native American artefacts, including ancient cliff dwellings and rock carvings.
Natalie Landreth, a lawyer representing the Native American Rights Fund, said: "We will be fighting back immediately. All five tribes will be standing together united to defend Bears Ears."
The group believes the cut would violate the Antiquities Act.
Jonathan Nez, vice president of the Native American territory Navajo Nation, said Mr Trump was ignoring the treaty rights of Native American nations.
He said: "It's a sad day in Indian country."
Around 3,000 demonstrators lined up outside the city's Utah State Capitol Building to protest the decision.
Some held placards which read "keep your tiny hands off our public lands" and chanted "lock him up".
"Some people think that the natural resources of Utah should be controlled by a small handful of very distant bureaucrats located in Washington," Mr Trump said. "And guess what? They're wrong."
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A smaller group in support of the US leader's decision also gathered at the state capitol. Some said they were in favour of potential drilling or mining that could create jobs.
But among the critics was Connecticut senator Richard Blumenthal, who said the order "sets a dangerous precedent that threatens environmental treasures everywhere".