As the midday heat radiated in Old Town Alexandria yesterday, about 100 protesters converged on Market Square to call for the closure of the Mirant power plant, a 56-year-old facility on the banks of the Potomac.

Wearing surgical masks and carrying handmade signs, they said the coal-burning plant is an antiquated facility that causes pollution and illness in nearby neighborhoods.

The plant's neighbors at the northern end of the city have complained for years of high levels of mercury and other contaminants in air and water and of a sooty substance that they say covers surfaces around their homes and gardens.

"It affects my son," said Brenda Shaw, 51, who raised her children a few blocks away. "It started when he was 1 -- his eyes burn and itch and swell up." Her son's symptoms stop when he is out of the area, she added.

"Our plants died," said Mary C. Harris, who lives in the Marina Towers apartment building, which overlooks the brick- and aluminum-siding-clad plant.

"Our window frames are being eaten away," she said, adding that she believes pitting on the aluminum frames is caused by sulfuric and hydrochloric acids emitted by the plant.

Atlanta-based Mirant Corp. operates four plants in the area, including three in Maryland -- in Montgomery, Prince George's and Charles counties.

Although the Alexandria facility has been cited for violations in the past, Steven Arabia, a spokesman for Mirant, said it was now "compliant with all the laws" and had recently installed emissions controls that make it "cleaner now than it's ever been."

"The coal that we use there has the lowest mercury content of any coal in the mid-Atlantic region," he said, adding, "This plant is absolutely necessary for the continued reliable electrical service to the nation's capital."

That last argument riles opponents, who note that Mirant produces power used in the District and Maryland, but not Virginia. Ernie Lehmann, an Alexandria resident and one of the rally's organizers, said the facility supplies a small fraction of the power used in those areas. The plant produces 482 megawatts of electricity, according to the company Web site. In contrast, Mirant's plant in Chalk Point, Md., produces 2,429 megawatts.

Arabia said that air quality was a "regional issue" and that problems couldn't be attributed to one source. "We can shut all our power plants down and you would have a relatively moderate impact on air quality, and possibly no impact," he said.

But late last year the City Council revoked a 12-year-old ordinance that had allowed the plant to operate indefinitely, as well as two special-use permits; the moves made the plant in violation of zoning, giving Mirant seven years to amortize its investments and close operations. In February, Mirant sued the city of Alexandria to stop the zoning changes. The suit is pending.

At the rally, City Council members Rob Krupicka (D) and Andrew H. Macdonald (D) spoke against the plant, as did a spokesman for Rep. James P. Moran Jr. (D-Va.). Afterward, protesters marched about 15 blocks to the Mirant plant.

Standing in the road between her apartment building and the facility, Harris, a 10-year resident there, said she understands the need for power plants. "But not this one," she said.

Clarke Herberet, left, and Pat Troy take part in a rally against Mirant Corp.'s Alexandria power plant.