Two US Christian volunteers were beaten and robbed by masked attackers they suspect were Jewish settlers while escorting Palestinian children to school.
Kim Lamberty and Chris Brown say they were kicked, beaten with a chain and had their possessions stolen close to the Maon settlement, near Hebron.
A settler spokesman said he had no knowledge of the incident and opposed any violation of the law.
Israeli police said they were investigating the assault.
Ms Lamberty suffered a broken arm and Mr Brown a punctured lung in the attack.
Both have filed a complaint with Israeli police.
The two are members of Christian Peacemaker Teams - a group that monitors conflict between Israelis and Palestinians in the Hebron area.
Whilst recovering in an Israeli hospital in Beersheba, Chris Brown told BBC News Online it had been "was an ambush - a premeditated attack".
"They threw a stone at my head which knocked me over and then whipped me with chains," Mr Brown said.
He said that Ms Lamberty had her passport, mobile phone and money stolen by the settlers.
He also said that harassment of Christian volunteers working with Palestinians in the area is common.
"They normally throw stones at us or fire their guns over our heads - but this is the most vicious assault so far."
Palestinians also complain of violent intimidation by Jewish settlers in the Hebron area.
Around 1,200 soldiers guard some 600 Jewish settlers living in an enclave in the heart of Hebron, home to 120,000 Palestinians - an area that has seen some of the fiercest fighting of the four-year-old Palestinian intifada or uprising.