AMPA, Fla., July 15 — Wearing masks and making obscene gestures at police cameras, about 100 people on Saturday protested a new security system that scans faces in the city's night life district to search for suspects in crimes and runaways.

"Being watched on a public street is just plain wrong," said May Becker, wearing a bar code sticker on her forehead.

Ms. Becker joined demonstrators in the Ybor City district on Saturday night, wearing a sign reading, "We're under house arrest in the land of the free."

One protester walked by a camera, gestured obscenely and shouted, "Digitize this!"

Other protesters wore gas masks, Groucho Marx glasses and other items to protest the FaceIt scanning system the police are using in Ybor City, a neighborhood that attracts thousands of people on weekend nights.

The cameras snap pictures of faces and compare them with 30,000 images in a database that includes runaways and wanted criminals. The system works by analyzing 80 facial points around the nose, cheekbones and eyes.

Tampa is the only American city where the police use the technology for routine surveillance, but Virginia Beach is seeking a $150,000 state grant for a similar system.

The Tampa police say the system, which has been used in Ybor City the last two weekends, has led to no arrests.