Suspected Maoists killed 35 people in a land mine attack against a bus in central India on Monday afternoon, a local police chief told CNN.
In addition, 30 injured passengers were transported to a health care center, Dantewada Police Deputy Superintendent Sanjay Sharma said.
At least 14 policemen and 13 civilians were among the fatalities, according to CNN-IBN, CNN's sister network in India.
The civilian bus was blown 20 feet into the air after hitting an explosive device on a road roughly 280 miles (450 km) south of the state capital of Raipur, CNN-IBN reported.
"Today we have seen the wanton disregard for life," Indian Union Home Secretary G.K. Pillai said, according to CNN-IBN. "This is a new dimension. I can only say that the targeting of civilians shows the Maoists are desperate."
The strike occurred one day before the Maoists were set to launch a 48-hour protest in five states in response to a new government crackdown against them, CNN-IBN noted.
Suspected Maoist rebels killed at least seven officials when they blew up an armored security vehicle in eastern India earlier this month, authorities told CNN. Suspected guerillas killed more than 70 federal police in a deadly ambush in April.
India regards the left-wing insurgents as its most serious internal security threat. The Maoists have claimed to be fighting for the dispossessed since the 1960s.
India's home ministry said earlier this month that nonprofit groups and intellectuals found helping the banned insurgents spread their ideology would be prosecuted under the country's laws.
But Human Rights Watch decried the warning as an attempt to silence political speech.
"The Indian government should think twice before trying to silence political discussion and demanding endorsement of its views on Maoist groups," said Brad Adams, Asia director for the U.S.-based rights group. - CNN's Sumnima Udas contributed to this report.