Congress of SA Trade Unions (Cosatu) Free State secretary Sam Mashinini said: "This is really bad for the province and the country and we call for a thorough investigation into this matter and anyone who is found to have misused his powers... must face the might of the law."
The union federation was "shocked and disgusted" by the death.
The secretary-general of the Pan Africanist Youth Congress of Azania, Lucky Khoza said: "We've said it before: that militarising the police will not yield any good results in fighting crime, but will only result in the police unleashing brutal force against the poor."
The National Union of Metalworkers of SA said the assault was a reminder of apartheid hit squads that used to "kill and torture our people in townships"
The regional secretary Andile Zitho said: "We are really worried that our country is gradually degenerating into a police state where the police are a law unto themselves."
The Democratic Alliance said footage of the killing, broadcast on SABC news, showed the ANC government was willing to use apartheid-era policing tactics to clamp down on service delivery protests.
MP Wilmot James said: "It's a deplorable and desperate act that shows just how badly this administration has lost its way.
"It's also completely unacceptable that the ANC government has now attempted to censor the broadcast of this story by writing to the Independent Communications Authority of SA."
Media Monitoring Africa welcomed the decision by the SABC to give prominence and attention to the crime, but was concerned about discrepancies in how the story had been reported.
The National Education, Health and Allied Workers' Union called on Police Commissioner General Bheki Cele to ensure those responsible for the crime were charged.
"This reckless conduct by the police is nothing new and the unions have experienced it first-hand," spokesperson Sizwe Pamla said.
African Christian Democratic Party president Kenneth Meshoe said the police had behaved like a "pack of hyenas".
According to the SA Municipal Workers' Union the incident was a reflection of the current state of South Africa's democracy.
Spokesperson Tahir Sema said the government needed to "rein in" the out-of-control forces and engage with them on how service delivery issues could best be resolved.
The Centre for the Study of Violence and Reconciliation said it raised important concerns about the standard of police training and the overall standard of leadership within the SA Police Service.
"The use of force needs to be recognised for what it is... and appropriate steps taken to ensure that members are able to uphold the appropriate standards for using force," it said in a statement.