Mr Sarwar said it was a “shame” he had learned of his demotion from Labour's Shadow Cabinet via twitter while he was representing the party during a health debate at Holyrood.
The sacking of Mr Sarwar, who was regarded as one of the party's most effective frontbench performers, will be viewed as a major setback for the moderates within Labour.
Mr Leonard defeated Mr Sarwar in bad-tempered and divisive leadership contest almost a year ago. Mr Leonard was seen as the left-wing candidate while Mr Sarwar was portrayed as the more centrist option.
Mr Sarwar said: “I’m deeply disappointed to no longer be Scottish Labour’s Shadow Cabinet Secretary for Health. It’s a shame that I came to know about this decision through Twitter, while I was leading for Labour in a health debate in the Holyrood chamber.
“I am incredibly proud of the work I did on behalf of NHS patients and staff, particularly the creation of a workforce commission, the campaign to end the pay cap, and the fight to protect local services from SNP cuts.
“For the sake of all the people who need a Labour Government, I wish Richard Leonard and his Shadow Cabinet all the very best for the future.
“I will continue to serve my constituents tirelessly from the backbenches, campaigning for issues close to my heart including the battle against racism, prejudice and hatred.”