Lines are rapidly being drawn in a debate about the amount of affordable housing to be included in the development of the giant space above the West Side’s Hudson rail yards.

On Tuesday evening, the city and state disclosed plans to build a series of commercial and residential towers around a large open space on a 26-acre site just east of the Hudson River, and pledged to designate up to 20% of the rental units on the site as “affordable housing” for about 20 years after the project’s completion. Additionally, hundreds of units of permanent affordable housing were pledged for two off-site Midtown locations owned by the city and state.

Almost immediately, elected officials, activist groups, and community members called the amount of affordable housing insufficient and criticized its temporary nature.

“This site must include permanent affordable housing,” the president of Manhattan, Scott Stringer, said at Tuesday’s public forum.

The state’s Metropolitan Transit Authority, which owns the site, is seeking a high financial return from the private developer eventually chosen to build on the site, and worries that a higher percentage of affordable housing would weaken the bids and deprive the authority of additional resources for transit, sources familiar with the project said. The extraordinarily high cost of building platforms over the eastern and western rail yards, which are divided by Eleventh Avenue, will likely be an additional financing challenge.

In a request for proposals that may go out as soon as the end of this month, the MTA is said to be seeking hundreds of millions of dollars as a sale price for the property, most likely from one bidder.

“We remain committed to finding a solution that both maximizes returns to the MTA and provides a wonderful benefit to the community,” a spokesman for the MTA, Jeremy Soffin, said.

An organizing director with the affordability group Housing Conservation Coordinators, John Raskin, pledged to fight for greater concessions from the state and city. “The affordable housing plan is totally inadequate for the needs of the community,” Mr. Raskin said.

Seeking to avoid a repeat of the failed West Side Stadium proposal for the site, various project stakeholders have held numerous closed-door meetings in recent months to try to iron out potential kinks before the project enters the public approval process.

People involved in the discussions said the city and state have now included many items community leaders had requested in earlier talks, such as a provision for a school within the complex and support for preserving the High Line elevated rail viaduct as park land.

On the question of affordable housing, community members and elected officials say they plan to lean heavily on the state and city, which hope to reach a consensus with elected officials before putting the project out to bid.

The speaker of the City Council, Christine Quinn, who represents the district, will be a key figure in discussions; the council must approve the final proposal.

A spokesman for Ms. Quinn said the council is reviewing the plans.