Palestinian witnesses say Israeli helicopter gunships and tanks have fired on protesters in the Rafah refugee camp. At least 22 people are reported dead and dozens of others wounded.
Palestinians say many of casulaties were children and teenagers, and accuse Israeli forces of preventing ambulances from northern Gaza from reaching the scene.
Israel says it is probing the reports, but says it is too early to determine what happened. Palestinian news wires are calling the attack a massacre. Television footage showed hundreds of Palestinians fleeing the attack site, many of them carrying wounded from the scene.
Palestinians say protesters were attacked as they marched toward a Rafah neighborhood (Tel al-Sultan) that has been under Israeli siege the past 36 hours. Tuesday, 20 Palestinians were killed.
Earlier, Israeli forces involved in the large-scale military operation in Rafah used loudspeakers to call on all Palestinian males over age 16 to come out waving white flags of surrender, or risk having their homes demolished by Israeli bulldozers.
Israel says its Gaza incursion near the Egyptian border is aimed at arresting or killing Palestinian militants and closing tunnels used to smuggle weapons from Egypt.
Information for this story is provided by AP