Sonko began the protest on July 30 after he was accused of fanning deadly violence between his supporters and the security forces.
He was admitted to intensive care at Dakar's main hospital, said one of his lawyers, Cire Cledor Ly.
The attorney, who said he went to the intensive care unit on Thursday, said Sonko "lost consciousness yesterday."
Another lawyer, Bamba Cisse, also said Sonko was in intensive care, while Sonko's Facebook page said he had been brought in on Wednesday evening "after falling ill."
Several government officials were approached by AFP but declined to give a response.
Sonko has been in hospital while under detention since Aug. 6, when he was admitted for emergency care. Details of his condition are unclear.
The 49-year-old has forged a wide following among Senegal's young through fiery attacks on President Macky Sall, portraying him as a would-be dictator and head of a corrupt elite.
On June 1, he was handed a two-year prison term following a trial on charges of moral corruption of a young beauty salon worker.
The conviction, he says, was a plot to make him ineligible to contest next year's elections.
Clashes erupted, leaving at least 16 people dead.
He was arrested on July 28 and three days later formally charged with fomenting insurrection, criminal association and harming state security, relating to incidents dating back to 2021. His PASTEF party was also ordered dissolved.
The government has expressed doubts that he is genuinely observing a hunger strike.