In some places, officials have already started carrying out cuts. For instance, King County in Washington State, which includes Seattle, stopped issuing new housing vouchers on Friday.

“Sequestration will result in some 600 fewer families in our local communities receiving crucial rental assistance over the next year,” Stephen Norman, the executive director of the county housing authority, said in a statement. “Because rents are so high, many of these families may, quite literally, find themselves out on the street.”

Members of Congress have indicated that they might give agencies more discretion in fulfilling the cuts, to help blunt their impact. But policy experts said that in the case of many low-income programs, budget cuts would necessarily mean fewer people get help.

“There’s no loose change in the cushions,” Ms. Crowley said. “Anything you take out of HUD is going to reduce services and cut programs. There’s just no fat there. There hasn’t been for a long time.”

Other programs that assist low-income families face similarly significant cuts, including one that delivers hot meals to the elderly and another that helps pregnant women. Policy experts are particularly concerned about cuts to the supplemental nutrition program for women, infants and children known as WIC, which provides food and baby formula for at-risk families.

It is considered one of the most effective social programs in government, reducing anemia and increasing birth weights. But up to 775,000 low-income women and their children might lose access to or be denied that aid because of the mandatory cuts, according to calculations by the Center on Budget and Policy Priorities, a nonprofit research group.

The start of sequestration, a policy never meant to take effect, has left both sides seeking cover, with many Democrats dramatizing the impact of the cuts and many Republicans playing them down.