HARRISBURG -- A politically divisive plan that would direct millions in tax breaks toward Pennsylvania cable giant Comcast Corp., allowing the company to move into a 60-story skyscraper headquarters in the heart of Philadelphia, received a public airing yesterday in the state Capitol.

And some Center City building owners, fearing the new tower would cut into their own business, have begun protesting the on-again, off-again proposal by turning off the lights in their office high-rises. More than 10 building owners are participating in the visual protest, dimming the city skyline's brightest buildings.

Led by the Center City Owners Association, opponents turned the lights off starting Thursday, and say they plan to do so until they win their battle. Liberty Place towers, Bell Atlantic Tower and the city's Mellon Center were among the darkened buildings.

Dozens of opponents to the Comcast proposal crammed into a tiny hearing room yesterday, listening to testimony before the Senate Finance Committee. That committee will decide whether the plan, allowing Comcast to take advantage of 15 years worth of tax abatements after it relocates its corporate headquarters, will be sent to the full Senate for consideration.

Advocates -- including Comcast, the proposed building's developer, Gov. Ed Rendell and, tentatively, the mayor of Philadelphia -- say the new building would help keep one of the state's biggest employers in Pennsylvania, adding new jobs and dramatically changing the city's skyline along the way.

The proposed building, called One Pennsylvania Place, would be the tallest between New York and Chicago.

Critics, mainly property owners and workers from the city's downtown, say the city's office vacancy rate is already too high. A new, subsidized tower would make a tepid rental market worse, driving down price or drawing away tenants who would prefer to do business tax-free, they say.

"It's draining resources from other towers," said Ray Griffin, a Philadelphia resident who works at the Shops at Liberty Place, a posh downtown mall. "It's going to cut down on floor traffic into the mall."

The tower, whose builders, Liberty Property Trust, have dedicated at least 40 percent of the rental space to Comcast, would be built on what's now a parking lot, at JFK Boulevard and 17th Street. But before the tower will be built, Liberty says it must receive a "Keystone Opportunity Improvement Zone" designation for the property to draw Comcast as a lead tenant.

Without Comcast, it won't build the $480 million tower, said John Gattuso, a senior vice president at Liberty. "The two are intertwined," he said, even though his company has previously built office towers without first securing a major corporate tenant.

The Keystone abatement program, created in the 1990s under former Gov. Tom Ridge, offers a decade or more in business tax breaks to tenants on such sites, and there are hundreds of them across the state, including more than a dozen in Allegheny County.

The program was conceived as one that would foster development on blighted or old industrial properties, which without the tax breaks tied to them would be unattractive to businesses. Some critics of Rendell's plan say giving money to a well-to-do company, so it can transplant its headquarters from one part of the city to another, is a misapplication of the Keystone program.