SIDI BURANA, Egypt—Anti-Gadhafi protesters are firmly in control of the eastern Libyan town of Baida and surrounding areas, according to Egyptian workers arriving in the Egyptian border town of Sidi Burana.

They showed flash-drive and cellphone videos with grisly pictures of what they said were captured pro-government mercenaries being viciously beaten in the city of Baida. One video showed a dark-skinned man, who the Egyptian workers said was a mercenary from Chad, being beaten to death. Another video showed what the workers...