Hundreds who were lucky enough to already be staying in places like Yosemite and the Grand Canyon faced a deadline Thursday to leave, 48 hours after the shutdown went into force due to a budget standoff in Washington DC.
"We grew up seeing pictures of it in books," said Clare Cogan from Cork, Ireland, on honeymoon with her husband Mohally but locked out of Yosemite National Park, shuttered since Tuesday.
"You know, the cars underneath those huge sequoia trees. That was America," she told the Los Angeles Times, which called the spectacular Californian park "an emblem of partisan divide."
The National Park Service closed its gates on its 401 sites as soon as the shutdown went into effect after midnight Monday, leaving visitors -- including many from overseas -- frustrated at park entrances across the country.
Tourists who were already staying in hotels, cabins and campgrounds inside national parks like world-famous Yosemite were allowed to stay -- but only for 48 hours, after which they were told to leave.
"Guests ... who are already checked-in can continue with their vacation plans, but they are required to leave by October 3 at 3:00 pm," said Lisa Cesaro of the company that operates over 1,000 rooms in the park, DNC Parks & Resorts at Yosemite.
The park, which was already partially closed in August due to a massive wildfire, is known around the globe -- especially for Yosemite Valley, where tourists flock to see landmarks including the spectacular El Capitan and Half Dome rock formations.
"We came all the way from England to climb and get to the top of El Capitan, but now we won't get the chance," Tim Larrad, a 52-year-old retired police officer from Worcester told the Contra Costa Times newspaper at his campsite.
"It's very disappointing. This climb was lifetime stuff. The trip took a lot of time to plan and prepare for," he said.
The Yosemite spokeswoman said that since Tuesday most activities, including horseback riding and bike rentals, had been suspended.
"We are continuing to provide retail, dining and limited transportation services for overnight guests in the park through Thursday," she said.
"If the shutdown continues, we will try to reschedule those who have upcoming reservations or cancel their booking and provide a refund," she added.
Some 715,000 visitors flock daily to National Park Service sites across the country, on average in October, according to CNN.
Other world-famous tourist attractions shuttered until further notice include the Statue of Liberty, Mount Rushmore, Yellowstone National Park, and the Alcatraz prison island in San Francisco Bay.
Joshua Tree National Park, two hours' drive east of Los Angeles, is famous for its bizarre cactus-like trees and weird rock formations, which draw climbers from around the world, especially at this time of year between the high heat of summer and the cooler winter.
It, too, is closed, with park rangers giving newly arrived visitors the bad news: they will have to wait until politicians in Washington sort out their differences on spending, before they can enter.
In the Grand Canyon, visited by 18,000 people a day on average at this time of year, spokeswoman Kirby-Lynn Shedlowski said the gates were closed from 6:00 am Tuesday.
"All recreational opportunities in the park, including hiking, biking, mule rides, visitor centers, they're all closed," she told AFP.
People who were in accommodations in the park on Monday night had been "given 48 hours to make additional arrangements, and then they have to leave the park," she added.
"We're just trying to maintain an orderly shutdown and closure of the park," she said, adding: "Having to turn anyone away is hard."