A federal jury today convicted a Philadelphia man in the brazen daylight robbery of a Center City luxury jeweler.

Nathaniel Williams, 21, was one of four men who prosecutors said held up Govberg Jewelers and escaped with nine pricey Swiss watches valued at $216,800.

Govberg operates a boutique store inside Boyd's the upscale clothier at 18th and Chestnut Streets.

Federal prosecutors said Williams entered the store about 2:30 p.m. on June 4, 2009 with a handgun. As an associate held a gun to the head of a plainclothes security guard, Williams sprayed mace into guard's face. Two masked men then stormed into the boutique and began smashing display cases with hammers. One of them grabbed a handful of Audemars Piguet watches from the case, according to court documents.

One of the masked men attempted to break into another display case but was only able to punch a hole in the glass, cutting himself. The men then ran from the store and fled in a dark green sedan.

Surveillance cameras captured images of the heist on video. Investigators were later able to track blood left on the second damaged display case to Raynard Powell.

Powell pleaded guilty in April and sentenced to 117 months in prison, said U.S. Attorney Zane Memeger.

Williams, who witnesses identified in a photo array, was found guilty of robbery, conspiracy, and firearms counts. He is scheduled to be sentenced on Dec. 16.

Williams and Powell were prosecuted under the federal Hobbs Act which provides harsher penalties for robberies that interfere with interstate commerce.

The two other men involved in the robbery have not yet been charged, a spokeswoman for Memeger said.

Contact staff writer Sam Wood at 215-854-2796 or samwood@phillynews.com.