"The fact that the incident happened during a press meet proves the threat to democracy and freedom of expression in the BJP rule," he said.
He alleged that Sangh parivar outfits were behind the incident and accused the BJP of promoting such groups.
The Centre should crush groups taking law into their hands with an "iron fist," he said in a statement.
Demanding arrest of those behind the incident, Stalin, also opposition leader in the state assembly, said such episodes should not recur.
The CPI(M), whose workers thrashed the two men before handing them over to the police, blamed the RSS for the incident.