, displayed a message saying all of its units are closed and directed visitors to the Interior Department, the parent agency of the park service and others like the Bureau of Land Management.
The shutdown "has made a really bad situation even worse for national parks," said Theresa Pierno, acting president of the National Parks Conservation Association or NPCA, an advocacy group.
"Park budgets have seen drastic cuts over the last three years. And shutting the park gates is really bad for American tourism and family vacations and all the businesses that depend on our parks staying open to spur the local economies," she told reporters in a conference call.
According to John Garder, the NPCA's budget expert, communities that depend on parks could lose $30 million a day due to the closures.
The park service is furloughing nearly 22,000 employees, who are among 800,000 federal workers being put on unpaid leave until a new funding plan is in place.
Park grounds, visitor centers, hotels, campgrounds, and roads -- except highways that run through the parks -- are closed. All programs and permits for special events are canceled. Those staying in overnight campgrounds and lodges have until 6 p.m. EDT on Thursday to leave the parks, according to the Obama administration.
Phil Francis, former superintendent of the Blue Ridge Parkway, said the shutdown coupled with automatic budget cuts that took effect earlier this year are hurting the park service's ability to protect the country's environmental and cultural treasures. The shutdown comes at a time of peak visitation at many parks, he added.
"It is irresponsible in my opinion that the world's richest nation cannot protect its most important assets," Francis said. "To have it happen in October is like closing a mall during Christmas time."
Polls show overwhelming support for national parks and forests.
Parks, passport services, clinical trials at the National Institutes of Health and a range of other services were closed due to the shutdown, after the Democratic-majority Senate rejected attempts by the Republican-led House to roll back or delay the Affordable Care Act, also known as "Obamacare."
Air traffic controllers, Border Patrol agents, most food inspectors and other employees responsible for public safety and national security showed up to work because they aren't subject to furloughs. The shutdown also won't affect programs like Social Security and Medicare that don't rely on annual congressional appropriations.
The health care law's implementation went on as planned. On Tuesday, online exchanges began enrolling participants seeking low-cost insurance plans.
In response to the Senate's Monday night rejection of the House's last-ditch attempt to avoid a shutdown and delay Obamacare for a year, House Republicans proposed on Tuesday providing funding for a handful of government programs like national parks, the Smithsonian museums in Washington and the Veterans Administration.
White House spokesman Jay Carney rejected the latest GOP ideas, calling them "not a serious approach."
Contributing: The Associated Press