"My respected father and your 'Netaji' is no more," Akhilesh Yadav said in a tweet from the Samajwadi Party's official account.
Uttar Pradesh Chief Minister Yogi Adityanath announced a three-day state mourning as a mark of respect to Samajwadi Party founder Mulayam Singh Yadav. "Uttar Pradesh government announces three-day state mourning on the death of Mulayam Singh Yadav. His last rites will be performed with full state honour," Adityanath said in a tweet in Hindi.
The SP leader had been under critical condition for the past few days.
"Mulayam Singh Yadav is quite critical today and is on life-saving drugs. He is being treated in the ICU of the hospital by a comprehensive team of specialists," said Dr Sanjeev Gupta, Medical Director at Medanta Hospital in Gurugram on Sunday.
Condoling his death, Prime Minister Narendra Modi said Yadav was a remarkable personality and was widely admired as a grounded leader.
"Mulayam Singh Yadav served people diligently and devoted his life towards popularising ideals of Jayaprakash Narayan and Ram Manohar Lohia," he said in a tweet.
Mulayam Singh Yadav has been under treatment since August 22 for his regular medical checkups and examinations.
Last week suddenly his health deteriorated after which he was shifted to ICU at Medanta Hospital.
Earlier on Friday, Union Defence Minister Rajnath Singh met the ailing former UP CM at Medanta hospital in Gurugram.
On Wednesday, Haryana Chief Minister Manohar Lal Khattar and Rashtriya Janata Dal (RJD) chief Lalu Prasad Yadav visited the Medanta hospital in Gurugram to check on the health of Mulayam Singh Yadav.
Born on November 22, 1939, Mulayam Singh Yadav is one of the most prominent leaders of Uttar Pradesh who served as the Chief Minister of Uttar Pradesh thrice and has also served in the Union Government as the Minister of Defence.
Fondly called "Netaji", or respected leader, by his supporters, Yadav became Uttar Pradesh's chief minister for the second time in 1993. He has been elected 10 times as MLA and 7 times as Lok Sabha MP.
