A federal law taking effect Monday may alter the standard checklist for many Americans as they pack to visit their national parks: insect repellent, snacks, hiking boots . . . double-barreled shotgun.

Visitors now can pack heat in any national park from Gates of the Arctic to Everglades, provided they comply with the firearms laws of the park's home state, according to the new law that was passed as an amendment to credit-card legislation.

In some instances, they may carry concealed and loaded firearms, including at campsites in Yosemite Valley, along trails at Yellowstone and at the rim of the Grand Canyon.

Gun advocates welcomed the law as overdue, saying firearms are allowed in national forests and other federal lands, many of which are next to national parks.

But opponents say guns don't belong in the nation's highly protected parks, where it remains illegal to fire a weapon or kill an animal and where employees, including most rangers, are unarmed.

The presence of guns, they say, could increase the chance of deadly accidents and up the ante in confrontations between park visitors or between visitors and wildlife.

The law, passed by Congress in May, reverses 94 years of National Park Service policy that generally allowed visitors to transport unloaded, disassembled weapons in the trunks of their cars. It applies not only to national parks, but also to national wildlife refuges.

Weapons won't be allowed in buildings where federal employees work, such as the Statue of Liberty and park visitors centers. But restaurants, hotels and gift shops will be subject to the new gun law. Yosemite's historic Ahwahnee hotel, for example, must allow visitors who are legally entitled to carry weapons to bring them into the building.

Gun-rights advocates are planning to mark the occasion by holding weapon-toting cookouts at Gettysburg, Valley Forge and other parks.

Bill Wade, chairman of the Coalition of National Park Service Retirees and a former park superintendent, said some of the changes will be nuanced.

"I don't expect that there's going to be hordes of people going into parks on Monday with rifles slung over their shoulders," he said.

Mike Stollenwerk, co-founder of OpenCarry.org, which advocates legally carrying holstered handguns in public, said the law opens 85 million acres of the country where gun owners say they have felt unwelcome.

"Now it's a big sea of gun freedom," Stollenwerk said. "The idea is that the same rule that applies on Main Street will apply to the national park. We will have a uniform federal rule that will make it easier for everyone to know the rule."

Critics, however, say there are as many potential complications as there are state and local gun laws.

David Barna, a National Park Service spokesman, said park websites are providing some guidance to visitors, but it is the responsibility of each gun owner to understand the laws of the state they are visiting.

That could confuse visitors at the more than 30 national parks that span more than one state. The Appalachian Trail crosses 14 states. And though Yellowstone is mostly in Wyoming, parts of the world's first national park also straddle the borders of Montana and Idaho, each of which has different weapons laws.

Implementing the law will be particularly hard in California, which has more national park units than any other state and some of the nation's most restrictive firearms laws.

State law generally prohibits possession of concealed and loaded weapons, although local law enforcement agencies can issue concealed-carry permits, and carrying a loaded weapon may be allowed in some unincorporated areas.

Officials are scrambling to fully comprehend how the law will play out at national park units in California, from Redwood and Presidio, to Death Valley and Joshua Tree.

"Many of the details of the law are unknown at this time," said Deputy California Atty. Gen. Alison Merrilees. "It won't be crystal clear the day the law goes into effect."

The park service is restrained from taking a position on legislation, but an array of law enforcement groups and retired park employees oppose it.

Critics fear that looser rules on guns could encourage wildlife poaching and increase destruction of historic or cultural resources. Shooting cactuses, signs and structures is common on other federal lands.

Opponents say the law ratchets up the potential for violence in parks, where for seven consecutive years rangers have been the most assaulted federal law enforcement officers, according to data compiled by the park service.

Major crimes reported in 2008 were at the lowest level since at least 1995, according to park service statistics. There were 1,844 weapons-related offenses reported in national parks last year.

Lesser offenses are on the rise, however, including the common offense of car clouting: breaking into vehicles left in parking lots for long periods.

"What do we do when a person does the right thing, and leaves his gun locked in the car when he goes into the visitors' center and someone steals the gun?" said John Waterman, a ranger at Valley Forge National Historic Park.

Bill Wade, of the Coalition of National Park Service Retirees, worries about law-abiding gun owners who may not be accustomed to camping in wild places.

"The single biggest threat is the situation that's going to occur in campgrounds, where you have inexperienced people, who don't know much about parks, who are out of their element," Wade said. "You are sleeping under the trees and you hear a noise. You look out of the tent and see a shape and start firing.

"It changes the dynamic in parks. People go there to feel safe and secure, to get away from the stuff they face every day where they live."