Neighbors of the eye-catching federal building at Seventh and Mission streets say its public plaza has become a lawless haven for the South of Market homeless, rather than the pleasant open space its builders intended.

When the federal building empties out at 5 p.m. on weekdays, street people know they can set up camp, deal drugs and do whatever else they want at night and on weekends, with almost no threat of retribution from building security, according to the residents of SoMa Grand, a swank 22-story condo building across from the plaza, who wrote a letter to federal authorities.

"The federal government spends hundreds of millions on improving neighborhoods throughout the country," said Claude Gruen, a SoMa Grand resident. "Part of that effort should be to simply maintain and police their own space."

Gruen and his wife, Nina, who led an effort to collect almost 170 signatures from building residents, said the unsavory and often illegal activity on the small plaza was dragging down any economic revitalization in the area.

"You really don't feel the place is at all pleasant," said Nina Gruen, who, along with her husband, runs an urban-development consulting business. "When it creates these kinds of negative problems for the neighborhood, they should be addressing it."

Federal authorities are aware of their neighbors' concerns and have already added extra security from the Federal Protective Service, the police arm of the federal government, said Traci Madison, a spokeswoman for the General Services Administration, which maintains the building.

"As part of a cyclical security reassessment ... additional security coverage has recently been added," she said in an e-mail. "We are now better positioned to have the grounds monitored from the building's security control center after hours and over weekends."

In a letter responding to residents' concerns, federal officials said they would also immediately begin cleaning the plaza on weekends because of its "highly public nature and intense public uses."

But the plaza's neighbors, who have been trying to get the attention of federal officials for months, fear it may be too late.

"If you let it go, it just gets worse and worse and worse - that's what happened here," said Gary Buckner, president of the building residents' association.

When the super-green federal building opened in 2007, Thom Mayne, the architect, hoped the plaza could become a hub for the improving the neighborhood, "offering much-needed open space and services to the local community," promotional materials said at the time.

But visions of farmers' markets have turned into something else, especially on three-day weekends when the building is shuttered for more than 72 hours, neighbors say.

"What you see there all day, 24/7, is people drinking, you see people urinating on the walls, you see everything," Buckner said.

Both the Gruens and Buckner said they anticipated frequent interaction with some of San Francisco's grungier residents when they moved to the neighborhood, but not to this extent.

"It has changed a lot," Buckner said. "We understood it was a transitional neighborhood but that doesn't mean it is OK for us to see people urinating and defecating when we walk out our door."