The one-day nationwide strike called by Greece's largest public sector union ADEDY is the first walkout since the Mitsotakis government's re-election for a second term.
The protesters marched to parliament, where lawmakers were debating the planned changes, which are expected to be passed this week.
It also enables employers to implement a six-day working week.
Employers face a fine up to 10,500 euros ($11,175) if they fail to declare an employee's extension of working hours or change of shifts.
The bill also introduces fines and a six-month jail term against those who obstruct employees from working during a strike.
Persons:
Louisa Gouliamaki, Kyriakos Mitsotakis, Lambrini Christoyanni, Renee Maltezou, Andrew Cawthorne, Barbara Lewis
Organizations:
REUTERS, Rights, Employers, Greece's Communist Party KKE, Thomson
Locations:
Athens, Greece