The explosions all took place in Coimbatore city in Tamil Nadu state, shortly before a rally by a Hindu nationalist leader, Lal Krishna Advani. Police said Mr Advani had cancelled his speech.
The bombs, in parked cars, went off at 15-minute intervals at locations including a hospital, railway station, town hall and near a bus station, police said.
"Fortunately, Mr Advani is three hours behind schedule." Mr Advani's Bharatiya Janata Party (BJP) said in a statement. "The death toll will certainly go up. This is just in one hospital," an official at the Coimbatore police headquarters said.
It was the most violent incident in the run-up to voting tomorrow for 222 of the 543 seats in the federal parliament's lower house, the Lok Sabha.
Paramilitary troops have been rushed to dozens of sensitive areas across the country, and at least two dozen have people have died in scattered violence in the troubled north-east, where several rebel groups are fighting for independence.
Dozens of militants in northeastern Assam have been detained, police said. Separatists have reportedly killed two people there since Friday. A left-wing candidate was killed lastweek. As many as 11,000 polling stations in Assam were declared security-sensitive after a Maoist group called for an election boycott. Reuter