The National Capital Planning Commission voted late on Thursday to approve the proposed 76 metre arch in Washington, after hearing from about 100 opponents including Democratic lawmakers.
However, US District Judge Tanya Chutkan, who is overseeing a lawsuit challenging the plans, issued a temporary restraining order early on Friday, blocking the Trump administration from breaking ground for 14 days.
The Justice Department had filed a notice with Chutkan informing her that a contractor had already done site preparation work on October 31, apparently without authorisation from the National Park Service
The department has said an initial six to eight months of construction focused on excavation and foundation work could begin within days.
Above-ground construction would start in May 2027 at the earliest, with the job expected to take 33 to 39 months in total.
The arch is among several costly projects Trump is pushing in Washington despite opposition from local officials and preservationists, including building a massive White House ballroom.
At an event on Monday, he said the arch would also serve as a military asset and would be able to house drones and snipers.
"It's right smack in the middle of everything, so very important for our military. But from a beautification standpoint, it'll be the most beautiful arch in the world," Trump said.
The arch would include an eternal flame held by a 60-foot-tall gold, winged female figure, flanked by gold flying eagles.
It would resemble the Arc de Triomphe in Paris, but stand about 25 metres taller, making nearly as prominent as the US Capitol.
D.C. City Council member Phil Mendelson, who also serves on the planning panel, said the project had not received required approval from Congress, nor had Congress appropriated any money for it.
Opponents told the commission the arch would destroy the solemnity of the nearby Arlington National Cemetery, where hundreds of thousands of fallen service members and military veterans are buried.
Meanwhile, the president signed a proclamation that he said would preserve the Oval Office and Cabinet Room as a national monument, his first official action after his party was swept aside in midterm elections.
But the action does not prevent future presidents from restyling the iconic office, which Trump has filled to the brim with gold flourishes, nor does it prevent a successor from rolling back the proclamation itself.
Trump himself has reversed national monument designations made by his predecessors.
"It will not be allowed to be touched by anybody, destroyed, ripped down by stupid people, in order to build something in a slightly different shape," the 80-year-old former real estate mogul said.
It was the third day this week that the president delivered remarks fixated on his pet projects in Washington.
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