The Sharia police, known as Hisbah, say the people were arrested at different times across the city of Kano in the past couple of days.
Kano state is one of the several states in northern Nigeria where Sharia law was re-introduced since 2000.
Sharia law is being implemented partially along with the country’s secular law.
The Hisbah spokesman in Kano state, Adamu Yahaya, told the BBC that all those arrested were Muslims and officers do not target non-Muslims because they are not bound by Islamic law.
He said some of those arrested told the Sharia office that they were eating because they did not sight the Ramadan crescent themselves while others feigned sickness, but the authorities found their excuses baseless.
The 80 were admonished and released because they were ''first-time offenders", Mr Yahaya added.
They were warned that if they were caught again, they would be taken to court. The Hisbah say they will continue with their patrols throughout Ramadan with a view to arresting Muslims who do not observe the fast.
The Ramadan fast is compulsory for all adult Muslims but some categories of people - such as those who are seriously ill - are not required to fast.