The National Park Service's Richard Trott places a sign barring visitors to the Lincoln Memorial in Washington on Tuesday. Reuters

WASHINGTON—The federal government's forced shutdown of vast swaths of its operations will send more than 800,000 federal workers home without pay, close national parks and cripple some programs, while leaving essential services up and running.

With no budget deal reached in Congress by the midnight deadline, workers were set to report to their agencies Tuesday morning for a half-day of shutdown preparations, including posting "closed" signs and securing computers. Workers were instructed to record voice-mail messages and post alerts on Facebook and Twitter notifying the public of closures.

Many furloughed workers would have up to four hours to wrap things up and then be sent home until further notice. They would be barred from using official smartphones or otherwise working, as long as the budget impasse persists.

At national parks, rangers prepared to advise most overnight campers that they have 48 hours to leave. All but essential roads across federal lands would be closed to traffic.

A Wall Street Journal review of agencies' shutdown plans found that more than 818,000 workers would be furloughed. In all, the federal government employs just under 2.9 million civilian employees.

The impact of a government shutdown varies widely by agency. Some, such as the Environmental Protection Agency, were sending almost everybody home. Others, like the Justice Department, said the vast majority of its workers were essential to protect life and property, and would stay on the job.

President Barack Obama laid out the impact in a late-afternoon appearance at the White House Monday. Social Security checks would be mailed, he said, and air-traffic controllers, prison guards and border-patrol agents would stay on the job. But other functions would cease.

"Veterans, who've sacrificed for their country, will find their support centers unstaffed," he said. "Tourists will find every one of America's national parks and monuments, from Yosemite to the Smithsonian to the Statue of Liberty, immediately closed."

About half of all Pentagon workers would be furloughed. Other agencies, such as the State Department and the Securities and Exchange Commission, have enough money left in most of their accounts to stay open for at least several days.

One program that won't be halted: the health-care law, which begins allowing people to sign up for insurance through the new marketplaces on Tuesday and which is at the center of the political fight. House Republicans have said they won't fund the government unless Democrats agree to delay the law. But in large part because the program has its own guaranteed funding stream, it will continue apace.

With the clock ticking and no budget deal in sight, federal agencies prepared to send at least 825,000 workers home without pay, and large swaths of the government were set to temporarily close. (Photo: AP)

Workers who are deemed essential and continue to work—"excepted" in the language of government—would be guaranteed back-pay after the fact. In past shutdowns, Congress has approved retroactive back pay for employees furloughed by government shutdowns. But given the cost-cutting zeitgeist of 2013, that appears less certain this time around.

Agencies spent Monday in a wait-and-see mode, hoping for action on Capitol Hill that would avert a shutdown. But there was some preparation, as employees prepared voice-mail messages notifying the public of a closure and documented the status of open cases so they could be resumed after a shutdown. Many agencies, including the Food Safety and Inspection Service, directed traveling employees to brace themselves for the need to return at the last minute.

Agencies and programs with their own funding streams, such as the Postal Service, Social Security and passport applications, won't be affected. Same for activities deemed necessary for human safety, such as meat, poultry and egg inspections, and guards at federal prisons.

But other impacts would be felt immediately. The Internal Revenue Service would cancel audit appointments, the National Institutes for Health would stop making grants, and the Smithsonian museums would be closed. The National Zoo would close, as would its popular "Panda Cam," showing the month-old baby panda and her mother.

Members of Congress were charged with deciding on an office-by-office basis which of their own staff members would stay on.

Of the federal workers set to be furloughed, about half of the threatened workers were from the Pentagon, most of whom were already required to take six unpaid days off earlier this year as a result of the automatic spending cuts known as the sequester.

On Monday, Sen. Barbara Mikulski (D., Md.) decried the looming disruptions and delays as she detailed the work of the Weather Service, the Social Security Administration and the NIH—all agencies with offices in her state that would be affected.

She noted that the NIH would have to cut off research, and that 70% of the NIH staff would be furloughed. "These are the people who are working on the cure for Alzheimer's. They are working on a cure for autism. They are working on the cure for arthritis. I'm just going through the 'A' words," she said.

NIH did say that employees will be kept on to care for its animal research subjects, including 1.3 million mice, 63,000 rats, 390,000 fish, and 3,900 nonhuman primates. "Many of these animals are priceless and have taken generations to breed," the agency's shutdown plan explains.

Others said a shutdown could show that many government functions weren't essential to start with. "It's just intuitive," said Bill Christian, director of government affairs at Citizens Against Government Waste, an organization that targets government overspending. "If you don't miss something when it's not working, you realize that it may not be a high priority for spending, particularly in such a dire fiscal environment."

—Andrew Ackerman, Alicia Mundy and Keith Johnson contributed to this article.

Write to Laura Meckler at