Activists Saturday sought to intensify pressure on City Hall and the NYPD for answers over a rookie cop's killing of an unarmed man, which police officials have labeled an accident, and whether crime-fighting tactics played a role in the Brooklyn tragedy.

Preaching Saturday at his storefront headquarters in Harlem, the Rev. Al Sharpton criticized the shooting of Akai Gurley, 28. Nearby was the man's grief-stricken family, including his 2-year-old daughter, and the mother of Eric Garner, who died after being put in an apparent chokehold during a July arrest in Staten Island.

"They're saying it was an accident. We're saying, 'How do we know until there is a thorough investigation?' " Sharpton said. "This baby will grow up without a daddy who did nothing wrong," Sharpton said. "This Thanksgiving these families will have to sit at a table with missing family members."

As Sharpton spoke, there were shouts of "murdered!" from the audience. Kimberly Michelle Ballinger, 25, the mother of Gurley's daughter, appeared stricken and looked down at the floor.

Gurley's sister, Akisha Pringle, who flew in from Florida, said, "All he is is an innocent guy walking down the stairs who was killed for no apparent reason at all going home to his baby mother and his kids."

Brooklyn District Attorney Kenneth Thompson has said his office would conduct a criminal probe of the shooting.

NYPD Officer Peter Liang shot Gurley in the stairwell of the Louis H. Pink Houses of East New York on Thursday. Police Commissioner William J. Bratton said it was an "accidental discharge" in a dark stairwell.

Bratton said Liang will have to explain why his gun was unholstered. Gurley was a "total innocent," the commissioner said. Liang, on the job less than 18 months, is on modified duty and, per procedure, has been stripped of his badge and gun.

Several hundred demonstrators chanting "no justice, no peace" marched Saturday night from the Pink Houses to a housing police station. "They can't just do this to our people," said Kenneth Walluyn, 58, who lives across the street.

"People should be able to walk in and out of their house and not get shot," said Argenys Taveras, 29, of Harlem. Mike May of Staten Island said "we need a political structure that has accountability." The march ended shortly after 9 p.m.

Sharpton's audience cheered Saturday as he lamented the darkened stairwells of housing projects as symptoms of municipal neglect. "They don't know if they're facing a cop or a robber when they're coming down the stairs," Sharpton said.

The shooting has focused scrutiny of the NYPD practice known as "vertical patrol," in which officers ascend and descend public housing project stairwells where many crimes occur. Critics are also questioning the commonplace use of rookies for such duty.

"The next step is an investigation and to deal with the vertical policing," Sharpton said. "Vertical policing in many ways is just up and down, up the stairs and down the stairs stop-and-frisk. We cannot have that."

City Councilman Jumaane Williams (D-Brooklyn) wanted to know why Liang had drawn his gun before encountering anyone.

"How is having your gun out with no provocation, your finger on the trigger, your safety off an accident? At minimum, that sounds like criminal negligence," Williams said.

Gurley's funeral will probably be after Thanksgiving, Sharpton said.

With Robert Brodsky

and Joan Gralla