Mohamed Badie, the top leader of Egypt's Muslim Brotherhood, suffered a heart attack while in jail but his condition has since stabilized, state run al-Ahram newspaper said on Saturday.
State-run news agency MENA denied a report by the private al-Nahar website, that Badie, 70, had died.
Badie, the Brotherhood's general guide, and many other leaders have been arrested in recent weeks in the toughest crackdown the Islamist group has faced.
Brotherhood spokesman Gehad el-Haddad said he had no information on Badie's health when asked to respond to reports that he had died in prison.
A medical team was sent to Torah prison on the outskirts of Cairo to assess Badie's medical condition earlier on Saturday, a security source told al-Ahram.
The source said his condition has stabilized and that the heart attack resulted from the "bad psychological state that he is going through".