Essebsi came to power in 2014 after the Arab Spring uprising which saw the despot of Zine El Abidine Ben Ali and sparked revolts in several Arab nations.
The African leader returned to the intensive care on Thursday after been hospitalised in late June over a severe illness. "Things are not going well," the leader's son Hafedh Caid Essebsi had told AFP on Thursday.
He had earlier promised that he would not stand in North Africa's country elections expected to take place in November.
Essebsi was born on November 29, 1962, and was the 5th President of the North Africa nation. He served the country in various capacities including Minister of Foreign Affairs, interim and later Prime Minister.
He founded the Nidaa Tounes political party and the party won a plurality in the 2014 parliamentary election and won the first regular presidential election after the revolution same year.