The alleged intimidation of general secretary Irvin Jim could be related to his views on nationalisation, says the union's president Cedric Gina.
"This is not the first time that something like this happened to him," Gina said.
"Irvin Jim is the man who represents the positions of Numsa on various issues.
"Who knows where this group that intimidated him came from? Numsa's call for the nationalisation of mines is a call that makes all types of people angry. Some right-wing organisations might want to take the situation into their own hands."
The National Union of Metalworkers of SA (Numsa) in a statement said Jim was followed by a "suspicious" car while leaving the University of Zululand on Thursday night. He was attending the SA Communist Party's 13th national congress.
However, SACP secretary-general Blade Nzimande on Saturday said a group of legitimate security for the eThekwini mayor followed the wrong car.
They thought they were following Durban mayor James Nxumalo, Nzimande told delegates at the SACP's congress.
"It was an honest mistake, they followed the wrong car."
He accused the media of sensationalising the incident and portraying KwaZulu-Natal as a "killing ground".
Numsa said Jim's security personnel stopped the car and confronted the occupants.
"When confronted, the occupants of the other car claimed that they thought the car belonged to… the mayor," spokesperson Castro Ngobese said.
"Surprisingly they did not know the name of the mayor, and even worse they could not produce authentic SA Police Service identification cards. The cars had false registration plates and were heavily armed.
"We call on the security authorities to provide the necessary safety and security because none of us can afford that a capable leader like Irvin Jim, whose only interest is defending the aspirations and needs of the working class and the poor, to perish simply because he has taken a principled class stance in our class divided society."
The African National Congress Youth League said the incident was "illegal and smells of suspicion and ill intent."
"Such actions of intimidation are a clear effort to suppress healthy politics and robust engagement within the tripartite alliance," spokesperson Magdalene Moonsamy said in a statement.
"This should be a concern to all in society… as such actions of intimidation weaken the safety net for those who raise their political views in society." – Sapa