M.K. Stalin, leader of the main Tamil Nadu opposition party the DMK, said police were guilty of "atrocities".
State Minister D. Jayakumar said in a televised address that it had been "unavoidable" for police to fire on protesters.
"Mass Murder of Innocent People", he tweeted Wednesday.
After meeting the injured in the hospital, Haasan demanded the closure of the unit and asked who ordered the police to open fire at the protesters. "Why were automatic weapons used to disperse the crowd and under what law is this permitted?"
Close Related Image Retired Madras High Court judge Aruna Jegadeesan to head the inquiry commission into the Tuticorin shootout. Close
Residents of the port city of Thootukudi, located at the tip of the Indian subcontinent, and environmentalists have been demonstrating for more than three months against the copper plant, one of India's biggest, alleging that it is a major source of pollution and a risk to fisheries.
Reports say that the police had initially tried to control the protesters with tear gas and baton charges as hundreds of protesters defied prohibitory orders on the 100th day of their agitation. A police officer at the site said: "Police had no option but to open fire because they would have ransacked the district headquarters". Congress president Rahul Gandhi termed the killings a "brutal example of state-sponsored terrorism". It added that the plant is now non-operational as the company was awaiting approval for it.
Tamil Nadu chief minister, Edappadi K Palaniswami, ordered the judicial inquiry into the shootings but defended the police. Even after heavy police force was deployed in the region, protests and violence continued.
"It is with great sorrow and regret that we witnessed today's incidents around the protest at Tuticorin", the firm said in a statement issued to media. Environmental activists say the cooper plant is polluting local water resources, allegations the company denied as "false propaganda". It has the capacity to produce 4,00,000-tonne of copper cathode a year.
Tamil Nadu is one of India's most prosperous and industrialised states but, as elsewhere in the country, environmental regulations are routinely breached and poorly policed. Reinforcements had to be rushed in from neighbouring districts to Tuticorin to assist the local administration in restoring normalcy.