The ruling on Friday by US District Judge Tanya Chutkan came a day after the US Interior Department announced plans to move forward soon with work on the 76-metre arch despite legal challenges and the lack of final approval from a government planning authority.
In a US District Court filing on Friday seeking a temporary restraining order, lawyers for a group of three military veterans and an architectural historian said the administration had "no legal basis for the construction work".
They said the arch violated federal law and needed approval from Congress.
The administration has argued a 1925 law that authorised a now-defunct commission to build Washington's Arlington Memorial Bridge also provides congressional approval for the arch - an interpretation opponents dispute.
Interior Secretary Doug Burgum announced on X on Thursday work was starting after "a very long wait" but the government said in a court filing on Friday the planned excavation work at the arch site was "not construction, or demolition in preparation for construction, of an arch".
The government said in its court filing it planned to dig four "test pits" to assess if any artefacts or cultural materials were present, starting on or after September 21.
It said it would restore the site by October 31.
Chutkan questioned Trump's authority to build the arch in April.
The government agreed in April to provide 14 days' notice before starting construction as part of a consent order entered by the judge.
The Interior Department reiterated that notice plan on Friday.
In her latest ruling, Chutkan ordered the administration to provide the court at least 48 hours' notice before "engaging in any activity" at the site, except efforts to gather information on archaeological resources beneath subsurface fill materials as required under the National Historic Preservation Act.
She also reiterated that any construction or demolition in preparation for construction before the National Park Service issues a final authorisation, or without providing 14 days' notice, would violate Friday's order and her previous consent order.
The arch is part of Trump's broader effort to leave his architectural mark on America's capital city, with projects including a new White House ballroom, renovation of the Lincoln Memorial Reflecting Pool and redevelopment of a golf course in East Potomac Park.
With a design evoking the Arc de Triomphe in Paris, the new monument would rise near Arlington cemetery, a sprawling military burial ground.
Opponents say the arch will destroy the cemetery's carefully designed historic sight line between the Lincoln Memorial and Arlington House across the Potomac River.
