Two bombs rigged to bicycles tore through a crowd of Muslim worshippers leaving Friday afternoon prayers at a mosque, killing 31 people and injuring 100 in what a top official called "a terrorist act".
Authorities clamped a curfew over the western city of Malegaon, which has a long history of violence between Muslims and Hindus, to stop revenge attacks. Past sectarian attacks have set off spiraling violence.
"There's a high alert across the state," P.S. Pasricha, Maharashtra state's director-general of police, told The Associated Press. "We have activated all police machinery to ensure that communal harmony is maintained."
Officials said the bombings were clearly intended to terrorize the city of about 500,000 people, which is 75 percent Muslim.
The office of Prime Minister Manmohan Singh said he "appealed for peace and communal harmony and has urged all citizens across the country to remain calm."
The explosions came as Muslims celebrated the festival of Shabe Barat, or the Night of Fortune, when they hold long prayer sessions seeking divine blessings, exchange sweets with neighbors and relatives and set off fireworks.
"It is a terrorist act. It is done by people who don't want peace," Maharashtra Chief Minister Vilasrao Deshmukh said at a press conference in Mumbai, the state capital. He said 56 of the 100 people injured were seriously hurt.
Both bombs were rigged to bicycles, he said.
"We found packets with the explosives attached to these bicycles," he said.
Pasricha told reporters that "the motive appears to be to create panic and make Hindus and Muslims fight with each other."
India has suffered a series of terror bombings over the past year, most recently carefully planned attacks on Mumbai's commuter trains in July that left more than 200 people dead. Those bombings were blamed on Pakistan-based Muslim militants.
Earlier this week, Singh warned that India may be facing even bloodier attacks.
"Reports also suggest that terrorist modules and 'sleeper cells' exist in some of our urban areas, all of which highlight the seriousness of the threat," Singh told India's state chief ministers during a conference on internal security.
The U.S. Ambassador to India, David C. Mulford, deplored the Friday attack.
"I am shocked and saddened by the brutal terrorist bombings that killed innocent civilians," he said in a statement. "There can be no justification for such heinous acts. The United States stands with India in its fight against terrorism."
Malegaon has been the scene of decades of religious violence, with riots between Hindus and Muslims most recently in 2001, when 15 people were killed.
India's bloodiest religious violence in recent years came in 2002, in the western state of Gujarat, set off by a train fire that killed 60 Hindus returning from a religious pilgrimage.
Muslims were blamed for the fire, and more than 1,000 people, most of them Muslim, were killed by Hindu mobs. India is about 80 percent Hindu.