Mayor Marion Barry unveiled a proposal yesterday to more than double the District's downtown convention space with a $400 million complex of exhibition halls, housing, university buildings and hotels in a six-block area north of Mount Vernon Square.

The complex, which would take several years to build, would be a mammoth undertaking for the quasi-public authority that Barry proposed -- and that the D.C. Council would need to approve -- to oversee construction. The underground part of the complex would stretch the length of five football fields, plunging seven stories deep to include vast exhibition halls and parking for more than 3,000 vehicles.

Above ground, running south from N Street NW, the project would include scores of units of new housing, the downtown campus of the University of the District of Columbia and a commercial complex of hotel space and offices.

The plan, as outlined by Barry, also includes the possibility of building a 25,000-seat sports arena on the site of the current Washington Convention Center, off the southwest corner of Mount Vernon Square.

A spokesman for the architectural firm that drew the plan for the larger convention center said a sports arena was only one option for the current center site, adding that any decision on that project is probably many months away.

Barry told reporters at a District Building news conference that because of an innovative financing plan for the new convention center space, D.C. taxpayers would have to bear none of the project's cost. However, the mayor and other city officials later retreated from that assertion, saying some city money would be used in the early stages for such items as land acquisition.

For example, buying the land for the two blocks where the housing part of the complex would be built would cost the city an estimated $10 million, said Wylie L. Williams Jr., deputy mayor for economic development. The D.C. government already owns the other four blocks.

Barry and Theodore F. Mariani, a local architect who designed the new complex, said the project would be financed with bonds issued by an independent authority created by the District government to oversee the new facility.

The bonds would be backed by the city's promise to lease the complex; revenue from parking and hotel taxes would go to the city to pay off the lease, and the debt for the entire project could be retired in 20 years, Mariani said.

Barry, who has made downtown development a priority in three terms as mayor, said the new exhibition space is needed if Washington is to remain competitive with other popular convention cities.

The Convention Center has 380,000 square feet of exhibition space, 250,000 of which are designated first-class space because of the vast size and amenities. The new complex would have at least 1 million square feet, 800,000 of which would be first-class, Mariani said.

Charlene Drew Jarvis (D-Ward 4), chairwoman of the D.C. Council committee that oversees the Convention Center operations, said she was pleased by the plan for a new facility, which she described as desperately needed because of space problems at the current center.

The center is running at nearly 100 percent capacity, forcing the city to turn away some convention business, Jarvis said. "I want Washington, D.C., to be the number one tourist attraction, and we can't do it with a second-rate convention center," she said.

However, Jarvis also said she expects the council to review provisions of the mayor's plan closely, especially the financing and housing components. She said the council "has to look carefully at the assessment that the whole complex will pay for itself."

Mariani said he believed the new convention center would go forward regardless of whether Barry is elected to a fourth term.

"Clearly the mayor has been the one stimulus to get it moving," Mariani said, adding the project enjoys the backing of a local hotel association, the Federal City Council and the Greater Washington Board of Trade. "But it's an idea whose time has come," Mariani said. "It will survive any future administration."

Staff writer Michael Abramowitz contributed to this report.