It took firemen more than 30 minutes to extricate the bodies from wreckage.
The dead were identified as 65-year-old P Pavai, her two daughters, R Selvarani (46) and R Geetha (42), G Vikneswaran (19), and A Akash (7). Pavai's 16-year-old granddaughter was critically injured.
According to Police, Vikneswaran apparently lost control of the car and it crashed into the signpost pillar.
"Police are still investigating to determine the cause of the accident," they said.