The 40-year-old, currently coach of the Chad national team, suffered a cerebral aneurysm and is in hospital in Yaounde.
Samuel Eto'o, Jamie Carragher and Craig Bellamy are among the former footballers who have tweeted words of support for the former Cameroon captain.
Song played a total of 34 times for Liverpool and 24 times for West Ham. He also played 137 times for Cameroon in a playing career that lasted over 16 years.