The 101/2-acre lot of soon-to-be vacant land near the heart of downtown Washington is the kind of prized property that city officials do not often have the chance to develop without limits. So when Andrew Altman, the city's chief planner, started dreaming about what to do with the site of the old convention center, he included residential units, retail and commercial offices, perhaps a hotel and, of course, hundreds of parking spots.

Then he considered the X-factor: What would draw people to the site and give it life, or, to use Altman's words, "create the kind of great public squares other international cities offer"?

His answer: a grand new public library.

After consulting his boss, Mayor Anthony A. Williams (D), Altman put together a bidding request for developers that reserved 50,000 square feet of space for the library, at an estimated cost of $150 million.

"It's one of those decisions that is historic in nature," Altman said. "We either do it now, or if it does not happen, it won't happen for another 100 years. This is a generational moment."

But if momentum is building to create a showpiece library for a revitalizing city, it could be squashed just as quickly. Soon after hearing of Altman's plan, a group of Library Board members, activists and architects balked.

Although they acknowledged that the city's flagship library, the Martin Luther King Jr. branch at Ninth and G streets NW, needs a major facelift, the patrons lined up in support of a different plan. They envision the city spending roughly $75 million to renovate the current building, which was designed in the 1960s by famed architect Ludwig Mies van der Rohe and has developed a cult following.

To this group, the loss of the old building would be tragic, the cost of a new building prohibitive and the idea of relocating the library absurd, considering the current branch is about 20 yards from the city's busiest subway station, Metro Center.

"It doesn't make sense to go for a smaller, new, more expensive building when you have an architectural gem already," said Gary Imhoff, who helps operate the watchdog Web site dcwatch.com.

Beyond the practical concerns of where Washington residents will read books or surf the Internet, the future of the library will go a long way toward determining how the city will shape its downtown center. On one side are city leaders who want to revitalize the city with a new "living downtown"; on the other are residents and activists who say the city should honor its traditions and preserve its heritage.

Opponents of a new library, such as Library Board member Alexander Padro, contend that Altman and other city leaders are motivated less by the public interest than the corporate dollar. By moving the library, they say, the city would be able to sell a prime block of land -- at the heart of downtown, a block west of the MCI Center -- to a commercial builder who would raze the original building and put up a high-rent office tower.

"This city has so many art treasures, but instead of making sure we honor that heritage, this administration feels like it should be sold to the highest bidder," Padro said.

City leaders scoff at such arguments, noting that a new public library at the old convention center site at Ninth Street and New York Avenue NW would take up valuable space that otherwise would bring in more money from developers.

Making matters even more complicated is the state of the city's 27-library system, whose book collections are incomplete and tattered, buildings are decaying and service hours are limited because of personnel and budget cuts. Just two weeks ago, library officials announced that because their budget was reduced by nearly $600,000, branches will be open only five days a week, instead of six.

Some patrons wonder whether spending millions to renovate or replace the main branch is appropriate in light of the woes at other locations. D.C. Council member Kevin P. Chavous (D-Ward 7), head of the council's Committee on Education, Libraries and Recreation, favors building a new library, but he acknowledged that the city must also address the branches.

"There's no question about that. For a lot of our citizens, the libraries are the main access to technology and the Internet," he said. "Branch libraries provide that."

That said, Chavous added that a new central library would be "a huge investment in the future benefit of our city."

City leaders said the same thing more than 30 years ago when they commissioned Mies, as he is commonly known, to use his signature modernism to replace the aging Carnegie library at Mount Vernon Square.

Mies delivered a boxy structure whose first floor is set back from the street, but whose top three levels of dark gray glass hang over the curb. Mies liked the glass because it opened the book stacks and the reading areas to view from passers-by.

If the outside of the building is more or less what the late architect had in mind, the inside has suffered over the years. In the summer, air conditioning condensation leaks through the ceiling into buckets carefully placed by staff. Some elevators, water fountains and bathrooms are out of service.

Budget concerns during construction meant that tan-colored brick was at times substituted for marble walls and floors. Central temperature controls don't work, meaning precious books and documents could be decaying. And Internet wiring is insufficient.

"It's very depressing," said Wendy Blair, head of the Literary Friends of the D.C. Library who supports the renovation plan. "It's a tragedy. It should be the best library in the country, but it's really kind of a sad place."

In the main foyer, bathed in yellow fluorescent light, an attendant operates the information desk, while patrons and homeless people sit on dozens of folding chairs set up in front of a television that plays cable news. A small, pine-wood book store is closed, the library unable to pay for a staffer. Old-fashioned card catalogues line the floor, anchored in place and too expensive to move.

In 2000, a team of seven architects delivered a pro bono report on how to renovate the building. Their proposal included creating a grand foyer and a sky-lit reading room on the second floor, replacing the gray glass with clear glass to brighten the building and adding a fifth-floor.

"The issue was: Could that building really be adapted to be state-of-the-art?" said Kent Cooper, an architect who worked on the project. "There's no question in my mind the building could work fine, but we really needed to doctor it."

Not long after the renovation plan was aired, Altman, who had been hired in 1999 from a similar post in Oakland, produced an opposing vision.

Altman had visited several other cities -- including Seattle, Phoenix, Denver and Nashville -- where new central libraries had helped revitalize downtown centers.

The libraries were modern, offering gift stores, cafes, theaters and even radio or television broadcast centers. To Altman, building such a structure at the site of the old convention center would draw people and bring life to an area that will also feature commercial and residential buildings.

"What animates a great public space?" Altman asked. "I looked around the country, and all these cities had major central libraries. There's a wave of rebuilding going on, using it as a part of a city's revitalization program to invigorate civic pride."

New buildings also can bring patrons back to the library system, said Judy Drescher, director of the Memphis library. "In our first 12 months and seven days we had 1 million visitors -- about double what it had been," she said.

Altman said he believes his plan is affordable through a combination of capital budget dollars and private funds. Library Board Chairman Marie Harris Aldridge and Director Molly Raphael support his plan.

Library board member Philip E. Pannell has called for public hearings. Others are pushing to register the Mies building as a historical landmark, which would make its destruction difficult.

George Ziener, former president of the Federation of Library Friends, hopes city leaders will strike a compromise and build a new library and preserve the current structure.

The building's "historical nature does not mean it has to remain a library," Ziener said. "It's an attractive building, but it's not functional as a library in the technological age. Don't confuse the building with the purpose."

Rita Thompson-Joyner, the assistant library director for lifelong learning, looks through some reference books at the Martin Luther King Jr. branch.Library Board member Alexander Padro says city leaders' plans to build a new downtown library are motivated more by money than the public interest.