Local agitators group had for long been demanding construction of a new dam in place of the 119-year-old structure, holding that it posed a grave threat to life and property of people in Idukki, where the dam is located, and four other districts in the Central Travancore region.
Shops remained closed in towns like Kumali and Chappath close to the dam area where the protesters had gathered in large numbers since yesterday.
Police said security had been stepped in border towns, especially at the road entry points through which inter-state buses and trucks pass through.
While major political parties refrained from backing the hartal, the ruling UDF had extended limited support to it within the Idukki district.
The Mahatma Gandhi, Calicut and Cochin universities have cancelled the examinations scheduled for the day.