Community groups and environmentalists yesterday assailed a proposal to build a transit center beneath six downtown blocks near Mount Vernon Square in Northwest Washington, calling the plan a "surprise attack" on D.C. residents that is aimed at making way for a controversial new baseball stadium.

Neighborhood activists said the 17-acre underground transportation depot, which could include 7,200 parking spaces and connections to buses, Metro and a planned trolley service near the District's new Convention Center, would pose a pollution hazard.

Critics threatened legal action to stop the $300 million transit project, which would be partially funded by the federal government. They called it an attempt to secure supplemental funding and speed groundbreaking for a stadium and other downtown entertainment venues.

"Let's call it what it is, a $290 million parking garage," said Stewart Schwartz, spokesman for the Coalition for Smarter Growth, which includes the Natural Resources Defense Council, the Sierra Club and the Audubon Society. "This is a subsidy for a baseball stadium, a subsidy for land and for parking for a baseball stadium," he said.

Douglas J. Patton, deputy mayor for planning and economic development, said the underground depot project was conceived before Mayor Anthony A. Williams (D) took office and is still under review.

"I don't want to defend this site saying it's the only one," Patton said. "There will be other sites mentioned."

Stadium and depot opponents held their news conference yesterday in the shadow of Museum Square Apartments, a 302-unit subsidized housing complex for disabled seniors that would be razed to make room for the center. They said the complex, east of Mount Vernon Square between New York and Massachusetts avenues NW, is being sacrificed to business interests.

"Even if a stadium is built, what good is it supposed to do for the people if nobody's living in the city?" said the Rev. Henry L. Anderson, 79, who has lived at the apartments for 14 years and is a minister at the Bible Way Pentecostal church across the street. "Will this be another Soweto where you will have to bus people to work?"

D.C. officials last week described the transit center as a tool to recruit private investors to an area anchored by MCI Center to the south and the Convention Center to the north. They said the depot would make it easier for visitors and commuters to come into the city without crowding downtown streets with vehicles.

The plan has been submitted to the D.C. Department of Public Works for a year-long study. The city also is looking at a more modest depot plan that would cost $75 million and contain 2,000 parking spaces.

The price tag on either plan does not include land acquisition, tenant relocation or transportation improvements.

In a report, "Choking to Death," released yesterday, transit center opponents said the depot would increase vehicle traffic and pump ozone and soot into the area, harming residents' health and elevating risks of asthma and cancer.

The report was done by Beth Solomon and J. Kirkland White, two members of the Committee of 100 on the Federal City, a planning group.

"We are appealing to the mayor to immediately put a stop to planning going on at the Department of Public Works," said Solomon, who owns a nearby high-tech media company and is a former Advisory Neighborhood Commission member.