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The United Action Front of Civil Society described the arrest, which occurred at Nnamdi Azikiwe International Airport, Abuja, as a "Gestapo-style" operation.
Ajaero was en route to the United Kingdom for the Trade Union Congress (TUC) conference when DSS operatives intercepted him.
CSOs demand immediate release of Ajaero
The civil society group expressed outrage at what they called a "needless show of force" and a deliberate attempt to incite chaos in the country.
The arrest of Ajaero, they said, signalled a worrying trend towards authoritarianism.
Hamisu Santuraki, the group's Head of Secretariat, issued a statement calling for Ajaero's immediate release and warning that his detention could spark widespread unrest.
"The DSS has, without reason, abandoned conventional practices in dealing with high-profile citizens like Comrade Joe Ajaero," Santuraki stated.
"Instead of issuing an invitation, they chose to dehumanise him, treating him like a common criminal."
Fear of national anarchy
The CSOs warned that civil disobedience could follow if Ajaero was not released.
"The abduction of Ajaero today is brash and indecent," the statement added.
"This act of rascality employed by the DSS has sent shockwaves across the country and threatens to lead to mass resistance."
As tension rises, many have expressed concerns that the arrest is a blatant attempt to silence dissenting voices within Nigeria's labour movement.