New Delhi, Nov. 20 -- It's 6 a.m. and Tholakele Nkwanyana is one of the first people to arrive at the Diepsloot public health clinic in Johannesburg, not to seek medical attention but to stop foreigners from getting care.

She and fellow members of South Africa's anti-immigrant group Operation Dudula - which means "to get rid of by force" - are dressed in military-style fatigues as they block the entrance and demand to see patients' identity documents. Mothers carrying children and others who are sick are turned away and told to go to private hospitals, which unlike public ones aren't free.

Similar scenes have played out at government-run clinics across South Africa's most populous province, Gauteng, as healthcare becomes the new battleg...