The lockdown which came into force early Friday will remain in force until the end of this month.
"The 12-day lockdown in Chennai and three other districts of Kanchipuram, Tiruvallur and Chengalpattu began today," an official said. "Only the essential services will be allowed in these areas."
Tamil Nadu has recorded the second-highest number of infections in India with 52,334 COVID-19 cases including 625 fatalities till date.
According to officials, shops will remain open until 2:00 p.m. local time during the lockdown and relaxations have been extended to medical and other essential services.
Health, revenue and police officials in Chennai, Chengalpattu, Kanchipuram and Tiruvallur districts have prepared an elaborate action plan to implement the full lockdown.
Public bus service has been suspended for quite a long time in the region, though flights and long-distance trains will continue as per the existing schedule of service providers.
Officials said the relaxations in other parts of the state will continue as per federal government directives.