Tunisia's 85-year-old parliamentary speaker becomes interim president, as nonagenarian Essebsi passes away
"Mohamed Ennaceur will take the oath in parliament" at 1500 GMT, Abdelfattah Mourou said, adding that under the constitution he was entitled to serve for up to 90 days.
Ennaceur - who at 85 is spritely compared to the Essebsi who died Thursday aged 92 - confirmed he would take up the post in an address on state television Thursday afternoon before being sworn in, calling on Tunisians to "unite in solidarity".
Essebsi was the oldest head of state after Britain's Queen Elizabeth II, came to power in 2014, three years after the Arab Spring uprising toppled the country's longtime despot.
Shrewd and wily, he was a long-time player on Tunisia's political scene, serving under Habib Bourguiba's regime, the father of Tunisian independence, as well as also under dictator Zine El Abidine Ben Ali.
His death ushers in a period of political transition in the North African nation, hailed as a rare Arab Spring success story.
Essebsi had been hospitalised with a severe illness in late June and was returned to intensive care on Thursday.
"Things are not going well," the leader's son Hafedh Caid Essebsi told AFP earlier on Thursday after he was readmitted.
Sources told The New Arab Essebsi's funeral is expected to take place on Saturday.
German Chancellor Angela Merkel on Thursday called Essebsi "a courageous actor on the road to democracy".
And Italian premier Giuseppe Conte took to Twitter to send his condolences for the passing of a "statesman of great ... humanity".
Concerned over a potential power vacuum ahead of the November polls, politicians and social media users had called for greater transparency about the president's health since he was hospitalised last month.
In April, Essebsi said he did not plan to stand for re-election in polls due to be held in November this year in order to make way for someone younger.
Presidential elections are scheduled for 17 November, after parliamentary elections which have been set for 6 October.
Tunisia's constitution, adopted in 2014, provides two measures in the case of a power vacuum.
The prime minister can take over the president's responsibilities for a period of no more than 60 days.
In both cases, the decision must be taken by a constitutional court after it validates the president's incapacity.If the vacancy is longer, the speaker of parliament is tasked with the role for up to 90 days.
But eight years after the Arab Spring, Tunisia has yet to set up a constitutional court.
Agencies contributed to this report.
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