The ballroom of the Brooklyn Bridge Marriott Hotel was filled to capacity last night, not with attendees of a wedding or bar mitzvah, but by those seeking below market-rate apartments that might not arrive for between four and 10 years — if ever.

In two information sessions for more than 2,500 invitees, an executive for the developer Forest City Ratner, James Stuckey, and a community activist, Bertha Lewis, explained the affordable housing opportunities that could come to Prospect Heights, Brooklyn, if the proposed Atlantic Yards project moves forward.

The $3.5 billion project would include a basketball arena and 16 towers containing more than 2,300 market-rate condominiums, 4,500 rental apartments, and office and retail space on 22 acres. Some of the development would be built over the Vanderbilt rail yards and some on land condemned using eminent domain.

Just over a year ago, Ms. Lewis, the executive director of the Association of Community Organizations for Reform Now, endorsed the Atlantic Yards project in exchange for the developer’s promise that half the apartments would be affordable.

Yesterday, Mr. Stuckey, representing the developer Bruce Ratner, told the predominantly black audience that the surveys placed on their seats were not applications for the affordable housing. “We want to find out more about what your needs are,” he said. “As we move forward with the development, we will be able” to contact you.

After sending out brochures, Mr. Stuckey said Forest City Ratner received more than 20,000 requests for more information, prompting it to organize last night’s event. A spokeswomen for the developer said it received more than 5,000 RSVPs.

The presentation is timed to coincide with the state’s pending release of the draft environmental impact statement, which would commence a public comment period. A final environmental review and state approvals are still required, but probable lawsuits aimed at the developer’s likely use of eminent domain could delay or even derail the project.

Critics of Atlantic Yards,who include neighborhood residents who say the project is too dense and would destroy the surrounding low-rise area, said yesterday’s meetings were a part of a public relations ploy to build political momentum for the approval process.

A spokesman for a coalition of opposition groups, Develop Don’t Destroy Brooklyn, Daniel Goldstein, said, “This is a luxury condo project with an arena tacked on, using some affordable housing component to get political support.”

Mr. Goldstein said the information session was closed to the public and open only to preselected invitees who replied to invitations from the developer. He said that because the project lacks approvals and financing, affordable housing is unlikely to appear on the site for at least four years, and even then the number of units would be a fraction of the 2,250 promised.

City Council Member Letitia James, an opponent of the project’s possible effects on the surrounding community but a supporter of affordable housing, said the turnout to last night’s event showed the dire need for affordable housing in central Brooklyn. But Ms. James said only 20% of the Atlantic Yards units would truly be affordable.

“Their definition of affordable housing is middle-income,” Ms. James said.

Last night, Ms. Lewis of ACORN said the Atlantic Yards affordable housing program was designed to meet the needs of a range of incomes, from low-income residents to middle-class. But she said she wished there could be more of it.

“If the government really wants to make things affordable for real, we would need more subsidies,” Ms. Lewis said. “They can fund wars; they need to fund affordable housing programs.”

The 2,250 affordable housing units would be interspersed throughout the apartment buildings, which all will offer doormen and other amenities including children’s playrooms, laundry rooms, and fitness centers.

“If an elevator works for the rich folks, it is going to work for the poor folks,” Mr. Stuckey of Forest City Ratner said. “Everyone has equal views.”

Mr. Stuckey would not say whether the number of affordable apartments would be reduced if the project was forced to downsize during the approval process.

Rents for the units would be based on the applicant’s income grouping, ranging from low-income families of four with a gross income as low as $21,270 to middle-income families with a gross income of $113,400. The rents would range from about $620 a month to $2,658 a month.

Residents would be selected at random from a pool of applicants and then checked for income eligibility. As required by city regulations, preference would be given to local residents, members of the police department, other city employees, and disabled applicants.

The parameters of the affordable units are designed to comply with government regulations that would allow the developer to qualify for additional state and city subsidies. Mr. Stuckey said those could include tax-exempt bonds and low-income housing tax credits.

The state and city have each promised to give $100 million to pay for infrastructure improvements to allow the Atlantic Yards development to move forward.