The Tskhinvali hospital where former Education Minister Alla Dzhioyeva is being treated said she suffered a "hypertensive crisis."
Officials of the separatist government and Dzhioyeva staffers say she suffered an attack of high blood pressure while she was being questioned during a police raid of her headquarters.
Irina Gagloyeva, a former separatist government spokeswoman who now heads a press center in Tskhinvali, told RFE/RL that the police action led to Dzhioyeva's collapse.
"Alla Dzhioyeva is in grave condition at the hospital now. She was taken there as a result of a criminal assault by law-enforcement personnel," Gagloyeva said.
Gagloyeva said the raid was conducted by armed law enforcement agents, who believed Dzhioyeva was "faking it" when she complained of feeling ill:
"Armed personnel burst into her headquarters and produced an unlawful invitation to the prosecutor's office," Gagloyeva said. "They didn't even bother to listen to her explanations."
Election officials declared Dzhioyeva the winner of the second round of the presidential election held in the breakaway region in November 2011, but those results were annulled by the Supreme Court at the request of her opponent.
The court ordered a new election for March 25 and barred Dzhioyeva from participating in it.
In December, Dzhioyeva negotiated an agreement with outgoing separatist leader Eduard Kokoity, who agreed to step down and fire several high-ranking officials of his government. But she annulled that agreement after the pro-Kokoity legislature failed to endorse the moves.
Dzhioyeva has called on acting de facto President Vadim Brovtsev to surrender power to her. She had planned to stage an "inauguration" ceremony for herself on February 10.
South Ossetia -- like the breakaway Georgian region of Abkhazia -- declared independence in the 1990s.
Following the August 2008 war between Georgia and Russia, Moscow recognized the two regions as independent countries.
Although a handful of other countries have also recognized their independence, the vast majority of the international community has not and still considers them part of Georgia.