The 94-year-old five-time Tamil Nadu chief minister was being continuously monitored and treated with active medical support, a press release issued by the hospital said.
"There has been a decline in the medical condition of DMK president and former Chief Minister M Karunanidhi. Maintaining his vital organ functions continues to remain a challenge considering his age-related ailments," it said.
"His response to the medical interventions over the next 24 hours will determine the prognosis," Executive Director of the Hospital Dr Aravindan Selvaraj said in the terse two-paragraph release, indicating that next 24 hours could be crucial.
Hundreds of DMK workers started marching towards the Kauvery hospital in Alwarpet in the heart of the city as news of deterioration of Karunanidhi's health broke out in the evening. Traffic in various parts of the city was paralysed.
Karunanidhi was admitted to the intensive care unit of the hospital on July 28 following a dip in blood pressure, which stabilised following medical intervention
In the last 10 days, a virtual Who's Who of Indian politics - right from President Ram Nath Kovind, Vice President M. Venkaiah Naidu, Congress President Rahul Gandhi, former Prime Minister H.D. Deve Gowda and leaders from various parties including the BJP, Left and others - visited the hospital to express their wishes for Karunanidhi's speedy recovery.
(With inputs from agencies)