June Weinstock, 51, may have suffered brain damage in the attack on Wednesday, in which a second US citizen was also injured by a mob in San Cristobal Verapas, 120 miles from the capital.
Ms Weinstock, a magazine editor, freelance writer and environmentalist from Fairbanks, Alaska, was attacked when she tried to photograph children. US Army reservists training in the area took her to hospital. 'She is in a delicate state. She is still in a coma,' her doctor, Regina Palma, said.
The US Army said the other US citizen, whose name was not given, was interpreting for Ms Weinstock when the attack took place. He needed medical treatment but was later released. Police said more than 30 people had been detained in connection with the attack.
Last month a New Mexico resident, Melissa Carol Larson, was nearly lynched by villagers accusing her of stealing a child. Police found no grounds to convict her. Despite no evidence of foreigners stealing children for organ transplants, many Guatemalans believe the rumours.
Local human-rights workers fear the attacks may be part of a campaign by the military to justify a greater role in internal security and undermine the work of international human-rights workers, journalists and diplomats.