The final rule is even broader than the version proposed in April, which would have allowed national park visitors to carry concealed weapons only in those states that allow such guns in their state parks. Now visitors will be able to carry concealed firearms into national parks in all states that have concealed carry laws -- even in states that outlawed such weapons in their own parks.

"If you can carry a concealed weapon down Main Street, you can carry it in a national park," said Interior spokesman Chris Paolino.

If a state wants to prohibit individuals from possessing a concealed weapon on federal lands within state boundaries, it may enact a law to do so, Interior said. The only two exceptions are Illinois and Wisconsin, which do not allow concealed carry.

Because the decision was made on the assistant secretarial level, it goes into effect immediately upon publication in the Federal Register, which is expected sometime early next week.

The move drew outrage from groups including the Coalition of National Park Service Retirees, the Park Ranger Lodge of the Fraternal Order of Police, the Association of National Park Rangers and the National Parks Conservation Association. They noted concerns raised by every living former director of the Park Service, several ranger organizations and retired superintendents and said the rule will put the safety of national park visitors and wildlife at risk.

But the department said the proposed change is intended to resolve conflicts between state and federal rules that complicate efforts to regulate firearms on public land. Other agencies -- the Bureau of Land Management and Forest Service -- defer to state laws on carrying firearms.

"The department's intent in adopting this final rule is to better reflect the decisions of the states in which parks and refuge units are located to determine who may lawfully possess a firearm within their borders, while preserving the federal government's authority to manage its lands, buildings and other facilities," an explanation of the rule says.

The new rule does not allow concealed weapons to be carried into federal buildings or facilities in national parks.

Interior Secretary Dirk Kempthorne ordered the department to review its firearms regulations in February after 51 senators -- including 13 members of the Energy and Natural Resources Committee -- sent letters claiming the policies were confusing and could conflict with the Second Amendment rights of gun owners.

The department broadened its proposed rule after receiving more than 125,000 public comments on it. Interior dropped a reference to "similar state lands" from the final version, saying it was ambiguous and confusing, and instead made a more general reference to state law as the governing standard.

Interior did not do a full environmental review under the National Environmental Policy Act for the rule. Instead, it used a "categorical exclusion," saying that the change was legal in nature and that no extraordinary circumstances existed to trigger an analysis.

The National Rifle Association, which argued that federal laws should keep in line with evolving state laws, took credit for leading the effort to amend the policy.

"These changes respect the Second Amendment rights of honest citizens as they enjoy our public lands," said Chris Cox, chief lobbyist for the NRA, in a statement. "We applaud the Interior Department's efforts to amend these out-of-date regulations."

Park, conservation and law enforcement groups oppose Interior's rule revisions, arguing that the proposal would reduce safety and boost the risk of impulse shootings of wildlife.

Bryan Faehner of the National Parks Conservation Association noted that in many states, individuals won't be allowed to carry a concealed weapon into a state park but will be able to in a national park.

"With this decision, state parks are going to have better protections for wildlife and visitors than our national parks," Faehner said. He added that N.P.C.A. is considering legal action but will first consult with other partner groups. Other groups also vowed to fight the change.

Rep. Raul M. Grijalva (D-Ariz.), who has been mentioned as a possible Interior secretary for the incoming Obama administration, wrote a letter during the comment period opposing the changes.

In explaining the rule, Interior cited an "alarming increase in criminal activity" on some Interior lands and the inability of federal law enforcement officers to guarantee a specific level of public safety on their lands.

But the parks and conservation groups said there were 1.65 violent crimes per 100,000 national park visitors in 2006, making national parks some of the safest places in the country.

"National parks are different from other public lands," said John Waterman, president of the Park Ranger Lodge of the Fraternal Order of Police. "The visitor population expects, demands and gets a higher degree of protection, enforcement and restriction in a national park. ... This vague, wide-open regulation will only increase the danger U.S. Park Rangers face."

Seven former directors of the National Park Service said in an April letter to Kempthorne that there is no need to change the existing regulation. "In all our years with the National Park Service, we experienced very few instances in which this limited regulation created confusion or resistance," the letter states. "There is no evidence that any potential problems that one can imagine arising from the existing regulations might overwhelm the good they are known to do."

The department disputed the argument that visitors with concealed weapons would be more likely to use them to injure wildlife, saying that has not been a problem in state parks. In response to comments that the rule would inhibit park rangers from halting poaching because brandishing a gun would no longer be probable cause to search for evidence of wildlife parts, Interior disagreed, saying it expects visitors to abide by prohibitions on poaching and that they would be subject to arrest if they break the law.

Interior gun rules were established in the 1930s to curb poaching. They were amended in 1983 to allow unloaded firearms to be carried in parks and refuges.