Critics of Duke Energy's planned coal-burning power plant near Charlotte filed a legal challenge Thursday seeking to overturn the state environmental permit for the facility.

Five environmental organizations contend that the permit issued by the Division of Air Quality in January would allow the Cliffside plant to exceed pollution limits set by state and federal clean air laws.

Duke began construction on the 800-megawatt plant Jan. 30, a day after receiving the state license. The Charlotte utility has 350 workers at the site in the Blue Ridge foothills and plans to bring in 1,600 workers at the peak of construction.

The plant is scheduled to begin generating electricity in 2012. Duke vowed Thursday to help the state defend the air permit against any legal challenge.

Opponents of coal-burning power plants hope that momentum is building to halt Cliffside, much like the 50-plus coal plants that were delayed or canceled last year amid rising costs and concerns about global warming.

Critics allege that the state permit should have imposed stricter emissions limits on mercury and dozens of other pollutants. They say Cliffside will increase haze at the Great Smoky Mountains National Park and contaminate waterways and wildlife with mercury, a potent neurotoxin.

"North Carolina illegally exempted Duke from clean air laws," said Stephanie Kodesh, a lawyer for the National Parks Conservation Association.

William Ross Jr., secretary of the Department of Environmental and Natural Resources, defended the permit, saying Thursday that it exceeded legal requirements for air quality.

A major point of contention is what pollution standards were in effect when Duke got the state's endorsement. Soon after the permit was issued, a federal court in Washington ruled that provisions of federal clean air standards apply to all power plants built after December 2000. The state did not impose those provisions on Cliffside. Had they been required, Cliffside would have had to meet much more stringent limits on mercury and other pollutants, critics say.

"The permit would allow 10 times more mercury pollution into the air every year than hazardous air pollution requirements would allow," said John Suttles, a lawyer with the Southern Environmental Law Center, which has led the fight against Cliffside for more than a year.

Ross said his staff is reviewing the court ruling to determine whether it requires any modifications in the Cliffside permit.

The five groups that filed the legal challenge also include the Environmental Defense Fund, National Parks Conservation Association, Sierra Club and Southern Alliance for Clean Energy.

They appealed the permit to the State Office of Administrative Hearings, an agency established to hear appeals of state government decisions, such as permits and employment decisions. An administrative law judge will issue a recommendation to another agency, the Environmental Management Commission, for a final ruling. Disaffected parties can appeal a commission decision to the Superior Court, the Court of Appeals and ultimately the state Supreme Court.

The goal of the state permit was to make Cliffside "carbon neutral" by keeping the company's total carbon dioxide output in North Carolina flat. Critics dismiss the carbon neutral claim as a shell game, because Duke had been planning to mothball the aging coal plants anyway. Cliffside would emit about 6 million tons of carbon dioxide a year, equivalent to about 1 million automobiles.

Last week, two environmental organizations filed their own challenge with the State Office of Administrative Hearings, including N.C. Waste Awareness and Reduction Network, which is based in Durham. This month, NC WARN filed a separate contention with the state Utilities Commission in Raleigh, asking for a moratorium on all new power-plant construction in the state.

The total cost for Cliffside had been estimated at $2.4 billion, including about $600 million for financing.

But this week, Duke learned from the Department of Energy and Internal Revenue Service that Cliffside qualified for a tax credit of $125 million for advanced coal-burning technologies.

That doubles Cliffside's previously announced tax credit of $62.5 million and trims the plant's price to about $2.3 billion.