Workers for the U.S. Agency for International Development who had been fired or placed on leave returned to their offices on Thursday to retrieve personal belongings, many still dumbfounded by the Trump administration’s sudden dismantlement of the 63-year-old aid delivery agency.

Hundreds of workers who just one month ago never imagined that they would soon lose their jobs en masse returned to the Ronald Reagan Building and International Trade Center in downtown Washington.

They were given just 15 minutes each to clear out their old desks.

The somber return came a day after the Trump administration revealed in court documents that it had completed a review of all U.S. foreign aid programs and was canceling nearly 10,000 contracts and grants, eliminating about 90 percent of U.S.A.I.D.’s work.

The agency’s annual budget of about $40 billion pays for the distribution of food and medicine, as well as disaster relief, disease monitoring, development work, and pro-democracy and civil society programs. Its work has been heavily concentrated in poor and developing countries in Africa and Asia.

Foreign aid makes up less than 1 percent of the federal budget.

In a joint statement, Democrats on the Senate Foreign Relations Committee denounced the canceled funding, calling the foreign aid review — mandated by an executive order President Trump signed shortly after taking office last month — “not a serious effort or attempt at reform but rather a pretext to dismantle decades of U.S. investment that makes America safer, stronger and more prosperous.”

“Ending programs first and asking questions later only jeopardizes millions of lives and creates a power vacuum for our adversaries like China and Russia to fill,” they said.

Trump officials argue that U.S. foreign aid programs have grown wasteful and detached from vital U.S. interests. Elon Musk, whose so-called Department of Government Efficiency led the initial charge against U.S.A.I.D., has wildly denounced the agency as “a criminal organization.” And Secretary of State Marco Rubio has claimed, without providing details, that the agency was dismantled so swiftly because its employees had subverted the administration’s efforts to review and limit their work.

Democrats and even some Republicans say the sweeping cuts to programs that were approved by Congress are unconstitutional. That is a key legal argument in a lawsuit against the administration brought by aid groups trying to stop the process. On Wednesday night, the Supreme Court handed the administration a partial victory, when Chief Justice John G. Roberts Jr. ruled that it did not have to meet an imminent deadline ordered by a federal judge to pay aid providers more than $1.5 billion for completed aid work.

But many larger legal and political issues remain unresolved. In their joint statement, the Democratic senators called for Mr. Rubio — a longtime supporter of many foreign aid programs who has disappointed some former Senate colleagues by presiding over and publicly defending the cuts — to appear before their committee.

“We expect him to not only consult with Congress but follow the law,” the statement said.

A crowd of supporters gathered outside the Reagan building, from which the aid agency’s signage has been removed, and applauded as people exited with their belongings. Some were in tears. Among them was Samantha Power, who led U.S.A.I.D. during the Biden administration, and who hugged stricken workers and supporters.

Because the agency’s workers were fired or placed on paid administrative leave so abruptly, and immediately barred from computer systems, email accounts and U.S.A.I.D.’s offices, most still had personal items at their former desks.

An email notice on Tuesday informed them that they would have limited time windows on Thursday and Friday to return to their desks. In language many found cold and even demeaning, it explained that packing supplies like boxes and tape would not be provided, that children would not be allowed on the premises, and that parking and transportation costs would not be reimbursed.

In what some recipients took as a message of extreme distrust, the notice also reiterated in detail the Reagan building’s list of “prohibited items,” which includes firearms, billy clubs, crowbars, nunchucks and dynamite.

In a statement on Thursday, David Miliband, the president and chief executive of the International Rescue Committee, said that his organization had lost at least 39 contracts and grants.

“We now face the starkest of stark choices about which services can be protected, and are calling on the American public, corporations and philanthropists to show that America’s generosity of spirit and commitment to the most vulnerable has not been lost,” Mr. Miliband said.