Varadan was brought from Vellore Central Prison to the Poonamallee Combined Court Complex for a hearing in a criminal case, police said.
A resident of Nesapakkam here, he is an accused in several crimes, including the murder of Kattupakkam Murugan in 2010.
Moments after he entered the court complex, two men pounced on him from behind and hacked him to death and escaped.
Policemen on escort duty were stunned by the sudden turn of events.
"We are working to secure the accused persons," a senior police official said but declined to elaborate.
Police said investigators had zeroed in on the culprits and the killing is suspected to be a revenge for the murder of a ruling AIADMK functionary "Bullet" Viswanathan of Nesapakkam.