At least 11 bomb blasts in quick succession ripped through the main city of Assam state and three other towns, wounding as many as 335, police said, according to news services.
At least 31 people were reported to have lost their lives in five explosions in the state capital, Gauhati. Many of the attacks took place in crowded markets in the state with bombs hidden in motorcycles or scooters, Reuters reported.
The largest bomb exploded near the secretariat -- the office of the Assam state's top government official, according to the Associated Press. Bystanders dragged the wounded and dead to cars that took them to hospitals.
"I was shopping near the secretariat when I heard three to four loud explosions. Windowpanes in the shops shattered and we fell to the ground as the building started shaking," H.K. Dutt, who was lightly wounded in the blast, told the AP.
Crowds of people angry over the blasts took to the streets of Gauhati later in the day, throwing rocks at vehicles and setting fire to at least two fire engines. Police imposed a curfew on the city and shut down roads leading in and out of the area.
No one claimed responsibility for the attacks. Assam has been a focus of a separatist insurgency for decades, but it has also recently suffered bomb attacks blamed on Islamist militants from neighboring Bangladesh. More than 10,000 people have died in separatist violence over the past decade in the region.
The bombings were condemned by leaders across South Asia.
"I am confident that the people of India will rise unitedly against these attempts to disturb peace and harmony and to destroy our social fabric," Indian Prime Minister Manmohan Singh said in a statement.
Thursday's attacks are the latest in a wave of bombing attacks to hit India in recent months, killing more than 125 people. Police have blamed most of those attacks on Muslim militants, although some Hindu militants have also been suspected of carrying out several attacks.
The Assam area also has been hit recently by ethnic clashes. At least 49 people were killed in July in violence between members of the native Bodo tribe and recent migrants to the area, most of whom are Muslims, Reuters reported.