The explosions occurred at around 11:00 a.m. local time (05:30 GMT) in Assam's capital, Guwahati, and the towns of Kokrajhar, Barpeta Road and Bongaigaon.
Local police believe the explosions were synchronized and carried out by the separatist United Liberation Front of Assam (ULFA) in retaliation for a recent clampdown on its activities in which many ULFA fighters were arrested or killed.
"No other group can trigger so many blasts in so many places in such a coordinated fashion," the media quoted Assam police chief R.N. Mathur as saying.
ULFA is a leftist militant group which was formed in 1979 and seeks to establish a sovereign Assam state via an armed struggle.
The Indian government banned the organization in 1990 and classifies it as a terrorist group, while the U.S. State Department lists it under "Other groups of concern."
However, any of dozens of militant separatist groups active in the region could be responsible for the Thursday wave of terror.
According to Associated Press, more than 10,000 people have died in separatist violence over the past decade in the isolated region bordering on Bangladesh, Bhutan, China and Myanmar.
