Hundreds of people gathered on Saturday at national parks from California to Maine to protest the Trump administration’s firing of at least 1,000 National Park Service employees last month.

A group called Resistance Rangers — consisting of about 700 off-duty rangers, including some who were fired from the National Park Service — tried to organize protests at each of the country’s 433 national park sites on Saturday to stand up against what they see as threats to public lands, including the job cuts. By the afternoon, there were protests at at least 145 sites, according to Nick Graver, a 30-year-old graduate student who helped organize the demonstration at Joshua Tree National Park in Southern California.

Protests were held in popular spots like Yosemite in Northern California, Acadia in Maine, Yellowstone in the Northwest, the Gateway Arch in St. Louis and Great Falls Park in Virginia, as well as lesser-known places like Effigy Mounds National Monument in northeastern Iowa. Tensions have been particularly high at Yosemite, where employees unfurled an upside-down American flag in protest across the face of El Capitan, a granite monolith.

Mr. Graver said his group was concerned not only about the firings but also about resource extraction on public lands and possible threats to national monuments, such as a proposal to remove the president’s power to designate national monuments.

The National Park Service said it was working with protest organizers to allow people to “safely exercise their First Amendment rights,” while protecting its resources.