Hassan passed away at a Karachi hospital today (June 13) after a prolonged illness and breathing complications, IANS reports.
His son Arif said: "My father has been ill for the last 12 years but this year his condition has deteriorated significantly... In the last one month, he was released from hospital for just one day before being admitted again."
Hassan, who has a huge fan following all over the world, had been suffering from lung, chest and urinary tract ailments for many years.
Prime Minister Gilani expressed grief and sorrow at the death of the world renowned singer. In a message, he said the singer was "an icon who mesmerised music lovers not only in Pakistan but also in the subcontinent for many decades".
Hassan, who last performed in India in 2000, wanted to visit the country again and expressed a desire to meet Dilip Kumar and Amitabh Bachchan; however he was unable to travel due to his deteriorating health.
Considered one of the most successful ghazal singers of Pakistan, Hassan was trained in classical music by his father Ustad Azeem Khan and his uncle Ustad Ismail Khan, who were both well-respected classical musicians.
In 2010, Sarhadein was released, featuring the duet 'Tera Milna' by Mehdi Hassan and Lata Mangeshkar, who once called him the "voice of God".
"Mehdi saab always wanted to sing with me. In fact, he sent me two to three recorded songs years back but he had to quit singing due to his prolonged illness. 'Tera Milna' is one of them," said Mangeshkar.
The legendary singer also played an important role as the cultural ambassador between India and Pakistan and visited his birth country several times.
Hassan leaves behind his nine sons and five daughters from two marriages.