The Interior Department has issued 251 offshore oil and gas leases at lower-than-ever royalty rates, a move hailed by industry as a spur to energy independence and denounced by conservation groups as "giving away the ranch."

Interior dropped the traditional 16.67 percent royalty rate to 12.5 percent, the minimum allowed by law, on 251 leases off the middle and southern Atlantic coast last year, according to department records.

This represents 15 percent of all off-shore oil and gas leases issued under the Reagan administration, Interior officials said. Royalties are charged by the government to capture portions of the profit from oil and gas development.

The lower royalties also will be used on lease offerings in the next year under Interior Secretary James G. Watt's accelerated development plans for the Outer Continental Shelf, the officials added.

The Carter administration, by contrast, issued no off-shore leases at rates below 16.67 percent, an In-terior spokesman said.

Interior defends the lower rates as a way to lure industry to explore deep waters, where development expenses are vastly higher than they are elsewhere.

However, conservation groups and some House Democrats have attacked the royalty policies and Watt's overall acceleration of off-shore leasing as an effort to balance the budget by selling public oil rights. Lower royalty rates tend to generate higher cash bids from oil and gas companies competing for off-shore tracts.

An oil company spokesman, who declined to be identified, said of the policies: "The majors probably wouldn't have any more incentive to explore with 12.5 than with 16 2/3. We still make our decision on the chance of finding oil."

An executive of Champlin Oil Co., a subsidiary of Union Pacific Railroad active in off-shore exploration, said lower royalties alone would not lure his middle-sized company into a high-risk venture.

Rep. Edward J. Markey (D-Mass.) recently introduced a bill to raise the minimum royalty on oil and gas leases from 12.5 to 16.67 percent. And a coalition of conservation groups, testifying yesterday before a House subcommittee, likened In-terior's off-shore leasing policies to "Napoleon's selling the Louisiana Territory to pay for his wars."

The row is the latest controversy to emerge in the debate over Watt's plans to offer 1 billion acres, virtually the entire Outer Continental Shelf, for oil and gas leasing in the next five years. A court challenge from environmentalists and coastal states delayed the program last year.

Oil industry spokesmen and congressional budget auditors recently criticized as overly optimistic In-terior's projection that the program would generate $18 billion in 1983 revenues, 180 percent of the highest previous year on record.

Interior officials have stuck by the projection, saying they expect high cash bids, spurred partly by the lower royalties, to make up the bulk of the projected $18 billion.