"mass organization" is a correct and usable phrase in written English.
You could use it to describe a group of people working together to achieve a certain purpose, such as in this sentence: The mass organization of volunteers helped distribute food and clothing to those most in need.
Till now, KMSS is a mass organization, and no mass organization can bring about serious change.
The Muslim Brotherhood, founded in 1928, developed from a mainstream Islamic reformist movement into a militant mass organization.
Each photograph in the series includes the red tie she wore when she was a Young Pioneer, the Soviet mass organization for children.
A network of mass organization, controlled by the state and the Communist Party, attempted to penetrate every sphere of private life.
The regime's prime minister and cabinet ministers have switched to civilian dress, transformed their mass organization into their political party, and are campaigning with the use of state properties, resources, funds and threats.
The following spring a National Charter proclaimed Egypt's regime to be one of scientific socialism, with a new mass organization, the Arab Socialist Union (ASU), replacing the National Union.
They linked together the many local governments and the mass organizations and determined their policies.
Only one legal political party, the Somali Revolutionary Socialist Party, and various socialist-style mass organizations existed.
The two parties competed for direction of nationalist policy, control of mass organizations, and recruitment of new members.
In their place, and mainly in order to mobilize public opinion, it created mass organizations in every sphere of activity.
The discipleship (striving toward God) that exists in a charismatic Christian community has nothing in common with the conformity (following orders) that characterizes modern mass organizations.
