Thilakaratne was set upon by an unknown gang in the Colombo suburb of Mount Lavinia, Superintendent of Police Ajith Rohana said.
The attack came after Thilakaratne had complained of death threats following a statement issued by him claiming interference on the independence of judiciary in the island nation.
His accusations prompted the government to issue a denial statement saying that the executive arm of the government was not at friction with the judiciary.
JSC in the recent weeks had initiated action against a judge for alleged impropriety and Thilakaratne's own personal conduct had been questioned by authorities.
Minster of Justice Rauff Hakeem condemned the assault on Thilakaratne.
Meanwhile Sri Lankan president Mahinda Rajapaksa had ordered an investigation into the assault on Thilakaratne.