President Trump is breaking ground on his 250-foot-tall triumphal arch near the Lincoln Memorial in Washington, even before a federal review panel has approved the project.

Doug Burgum, the interior secretary, on Thursday announced plans for the excavation work.

“After a very long wait (over 125 years!), we are preparing to start, over the next two-week period, the excavation work necessary for the Great Triumphal Arch” he wrote in a social media post, adding: “This will be one of the Great Pieces of American Architecture, honoring the history and significance of Arlington Cemetery and befitting the most powerful Capital in the World.”

The rush to push ahead with work on the arch follows a familiar pattern for Mr. Trump. He has frequently embarked on large-scale changes to historic properties in Washington without approvals from Congress or anyone else. The president last year abruptly tore down the historic East Wing of the White House after saying he would not touch the building.

In this case, Mr. Trump has proposed building a 250-foot triumphal arch near Arlington National Cemetery that he has said must be taller than the Arc de Triomphe in Paris. He has said the arch is meant to celebrate the 250th anniversary of American independence, though he has also said that the arch is for himself.

But Mr. Trump’s plans have not received final approval from the National Capital Planning Commission, which has authority over building projects on federally owned land in Washington. Nevertheless, the administration has been pushing forward with plans to build the arch. The administration has done preliminary testing of the site and has awarded $500,000 to an Oregon-based firm for an environmental assessment.

Mr. Trump last month named Mark Paoletta, a lawyer in his administration and a close associate of Justice Clarence Thomas’s, to lead the planning commission reviewing the arch.

“This project is not even close to being ready for prime time,” said Representative Donald S. Beyer Jr., a Virginia Democrat whose district includes nearby Arlington National Cemetery. “The administration has not won the legal battle over the proposal, which is plainly illegal without congressional authorization.”

Mr. Beyer said Mr. Trump and Mr. Burgum were “disregarding the risks and rushing this towering monument to Trump’s vanity because they believe Republicans will get crushed in November.”

The project is facing a legal challenge. A group of Vietnam War veterans has sued to stop construction, citing the lack of congressional approval for the project and arguing that the arch would obstruct the view between the Lincoln Memorial and Arlington National Cemetery. In court, the Trump administration has pledged to provide at least 14 days’ notice before starting construction.

But a recent Supreme Court ruling, which allowed construction of Mr. Trump’s planned ballroom to proceed, means that preservation groups could have a hard time achieving the legal standing to sue. Democrats have suggested that if they retake the House after the midterm elections, they could sue to stop the president’s construction projects that have bypassed Congress.

The Federal Aviation Administration, which has studied the arch project to assess its impact on a busy flight path in and out of Washington, has recommended that red, blinking obstruction lights similar to the ones atop the Washington Monument be added to the design — a common measure for large structures that go up near airports.

Mr. Trump has not said whether he would follow that safety recommendation.