Washington is bracing for a far-reaching federal government shutdown that could stop everything from tax refunds to local trash collection, the writing of some parking tickets and Saturday’s annual Cherry Blossom Parade.

As negotiations continue between President Obama and congressional leaders at the White House in hopes of a last-minute deal, federal employees will begin learning more Thursday about their work status and agencies will begin briefing contractors, federal worker union leaders, and state and local governments about the potential impact of a shutdown, Office of Management and Budget Deputy Director Jeffrey Zients said.

“We’re taking these steps because responsible management demands it,” Zients said Thursday.

Zients rattled off a list of government services expected to cease, including Smithsonian museums and national parks, and the processing of some IRS tax refunds and small business loans. But air traffic control duties, the U.S. Postal Service, FEMA disaster efforts and the National Weather Service would continue, Zients said.

He said there will be “many fewer staffers at the White House.”

Asked about the length of a shutdown, Zients said, “Obviously the shorter the better.” He said there was no estimate on how much it would cost the government to restart operations.

Messages being sent by senior agency leaders are asking workers to update pay and contact information before Friday, when individual employees would learn whether they have to work during a shutdown.

Nationwide, about 800,000 federal employees and hundreds of thousands of contractors could be furloughed, some deprived of their BlackBerrys and other devices, according to senior Obama administration officials familiar with the plans.

Any shutdown of the federal government, the chief industry of Washington, would also affect tourists, the Mall and its museums, and thousands of D.C. residents who would lose city services.

Ford’s Theatre, a national historic site, would be closed to the public and its programming canceled. The Washington Monument would also be closed.

Many government Web sites would stop updating information. But current Medicare, Medicaid and Social Security beneficiaries would continue to receive payments.

As congressional negotiators pushed last-ditch efforts to agree on a 2011 budget and prevent a shutdown, the National Cherry Blossom Festival announced that its annual gala parade, scheduled for Saturday along Constitution Avenue, will be canceled if lawmakers do not reach an agreement.

Even with the bleachers in place and parade-goers en route, festival officials said late Wednesday that the National Park Service could not honor the group’s parade permits if a shutdown occurs.

The Interior Department, which oversees the National Park Service, said in a statement: “Visitor activities that require a permit, including public events, will not be allowed or will be canceled or postponed. Visitor centers will be closed and access to park areas denied, including the Statue of Liberty and Ellis Island, Independence Hall, Alcatraz, and the Washington Monument.”

The shutdown will begin on Saturday morning if no budget agreement is reached.

The District would be hit hard.

The city, considered in a shutdown to be a federal agency, would face $1.5 million to $5.5 million in losses per week.

Along with trash collection, most parking enforcement would be suspended, and D.C. libraries and Department of Motor Vehicles offices would be closed.

Street sweeping would be suspended for the duration of a shutdown, city officials said. Trash pickup could resume, because federal shutdown laws permit government services that deal with property and public safety, they said. But that wouldn’t happen for at least a week.

“That’ll be a treat, won’t it?” Mayor Vincent C. Gray (D) remarked sarcastically about possible piles of trash around the city.

He said he thinks the potential shutdown is more evidence of the unfair treatment of the District by Congress.

“This is a concrete example of what it means to be treated like a second-class citizen,” he said.

Public and charter schools would remain open, but the University of the District of Columbia would close.

Police, firefighters and emergency medical technicians would remain on duty. The Board of Elections and Ethics would maintain its staff to prepare for the April 26 special election.

A key question for hundreds of thousands of Washington area federal workers is whether they would have to work through an impasse — without pay. Departments began answering such questions on Wednesday.

In an e-mail, Health and Human Services Secretary Kathleen Sebelius said she and President Obama “are very much aware that a shutdown would impose hardships on many employees as well as the groups and individuals our department serves.”

But “prudent management requires that I plan for an orderly shutdown should Congress fail to pass a funding bill.”

In messages sent to all employees, administration officials promised to inform people of their fates no later than Friday. But employees would be expected to report to work Monday to assist with any shutdown-related tasks, senior administration officials said.

When they arrive Monday, workers with government-issued BlackBerrys and other devices would have to surrender them to agency bosses.

At the Education Department, 4,150 out of 4,465 full- and part-time employees would be furloughed under a shutdown, according to an agency contingency plan. And federal statistical agencies, including the U.S. Census Bureau, would close on Monday, meaning that economic indicators scheduled for release would not be available. National security and law enforcement functions would continue at the Justice Department, but civil litigation and outreach to crime victims would stop or be curtailed.

Rep. James P. Moran (D), whose Northern Virginia district is home to thousands of federal employees, has scheduled a town hall meeting with federal workers for Thursday evening at Francis C. Hammond Middle School in Alexandria.

Smaller agencies would close almost completely. At the Government Accountability Office, a federal watchdog agency, a spokesman said that about 1 percent of personnel would be allowed to work.

Overseas, the overwhelming majority of U.S. diplomats are considered essential personnel and are expected to remain at their posts, according to officials familiar with the plans. Exceptions include lower-ranking administrative embassy staff members, many of whom are not American. Many U.S. Agency for International Development operations are expected to continue as normal, because the agency relies on many contractors.

Several fee-based government operations and agencies funded through multi-year budgets are expected to stay open, including the Veterans Health Administration, elements of the U.S. Patent and Trademark Office, and the Federal Highway Administration. Energy Department employees would have to report for work as scheduled, because that agency maintains enough funding not linked to the fiscal calendar to continue operating for at least a few days.

In the federal courts, trials would continue and lawsuits could be filed, said David Sellers, spokesman for the Administrative Office of the U.S. Courts.

If a shutdown lasted longer than two weeks, payments would be delayed for jurors and for lawyers appointed to represent indigent defendants. “You won’t have any contracts moving ahead, new equipment, advances in technology,’’ Sellers said. “The behind-the-scenes things that support the courts.’’

Meanwhile, Saturday’s Cherry Blossom Parade could feel the earliest effects of a shutdown.

“The word came down from the National Park Service officially that they will not be able to honor our permits if the government is shut down,” said Diana Mayhew, president of the National Cherry Blossom Festival, which concludes this weekend.

As many as 500,000 people normally attend the parade and the festival’s closing weekend, festival officials said.

“There are people coming from Japan to participate in the parade,” Mayhew said, as well as 14 marching bands from across the country.

“There are people coming to watch the parade,” she said. “What a shame that would be [for] those people, as well as the thousands of people participating in it who saved and earned their money all year round to be in this parade.”

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Staff writers Nick Anderson, Peter Finn, Al Kamen, Lyndsey Layton, Carol D. Leonnig, Jerry Markon, Steven Mufson, Lisa Rein, Nikita Stewart, Michelle Singletary, Carol Morello, Karen Tumulty and Brian Vastag contributed to this report.