Overriding protesters who greeted his motorcade’s arrival with chants of “F*** you Trump,” President Donald Trump announced in Utah that he would to drastically shrink a pair of wilderness national monuments - the biggest rollback of public protected land in US history.

In an appearance at Utah’s state Capitol in Salt Lake City, the President confirmed his decision to reduce Bears Ears National Monument from around 1.5 million acres to some 228,000 acres and to cut the Grand Staircase Escalante National Monument to one million acres, about half its current size.

It marked Mr Trump’s latest effort to unravel the legacy of his predecessor, Barack Obama, who in 2016 invoked the Antiquities Act to designate Bears Ears a national monument. Mr Obama and supporters heralded the designation as a way to protect a stunning natural landscape of towering rock formations and winding canyons.

But Mr Trump blasted the move as having “severely abused” the Antiquities Act, saying in a speech that Mr Obama had empowered “faraway bureaucrats at the expense of the people who actually live here, work here, and make this place their home”.

A press release announcing the change touted economic benefits like increased logging, hunting and “in some cases mineral development,” trumpeting “economic growth and prosperity, especially in rural communities” and a boost to local tax revenues.

“We have seen harmful and unnecessary restrictions on hunting, ranching, and responsible economic development,” Mr Trump said in his speech. “We have seen grazing restrictions prevent ranching families from passing their businesses and beloved heritage on to the children — the children that they love”.

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Numerous Congressional Republicans, including some who represent Utah, echoed those justification in lauding Mr Trump's move. Former Utah Rep Jason Chaffetz of Utah said in an op-ed that “we can build bathrooms and fire pits, and accommodate hunting, fishing, grazing, and permit accessibility without destroying the land” and heralded “responsible resource extraction”.

Mr Trump signed two proclamations after his speech. One would reduce the 1.3-million-acre (0.5 million hectare) Bears Ears National Monument by more than 80 per cent split into two areas.

The other would cut the state's 1.9-million-acre Grand Staircase-Escalante National Monument, designated by President Bill Clinton in 1996, nearly in half. The landscape of canyons, ridges and rock formations would be split into three zones.

A section of Bears Ears National Monument near Blanding, Utah on October 27, 2017 ( REUTERS/Andrew Cullen) (REUTERS/Andrew Cullen)

While a handful of monuments have been resized in the past, none has been cut back to such an extent, putting the president's proclamation in uncharted legal territory. Previous presidents including Woodrow Wilson and William Howard Taft reduced some monuments but were never challenged in court.

Mr Trump will ask Congress to look at the areas that are being removed from the current monuments to consider legislation designating some as a national conservation or national recreation areas, and create a co-management structure for tribes, an administration official said.

But where Mr Trump and his allies see an economic boon, critics warn of untrammelled development that could lead to mining and drilling on public lands.

Conservationists, Democrats and Native Americans who consider the area sacred blasted Mr Trump’s move, saying he was sacrificing a pristine space. The Navajo Nation has already said it plans to challenge Mr Trump’s action in court.

“The Bears Ears Monument is of critical importance, not only to the Navajo Nation but to many tribes in the region,” Navajo Nation President Russell Begaye said in a statement. “The decision to reduce the size of the Monument is being made with no tribal consultation. The Navajo Nation will defend Bears Ears”.

Jonathan Nez, vice president of the Navajo Nation, said: “It's a sad day in Indian country”.