An indefinite curfew has been clamped in the areas in the jurisdiction of Dhekiajuli, Thelamara, Rangapara and Biswanath Chariali police stations.
Meanwhile, intelligence sources said that the Assam police were given warning about the attacks.
More than 24 people were gunned down in Biswanath Chariali, six were killed in Dhekiajuli. A total of around 55 people, including four women, were killed and 10 others injured in a series of attacks by NDFB (Songbijit) militants at four places in Sonitpur and Kokrajhar districts of Assam.
The northeastern state has witnessed many such bloody attacks by insurgents in the past. The Tuesday's attack was in retaliation for the state's offensive against them.
On Tuesday, the militants of National Democratic Front of Bodoland attacked four different places in an hour.
According to police, the villagers told them that the attackers came on foot, armed with assault rifles and wearing military uniforms. They asked the villagers for water and suddenly opened fire using their AK-47 rifles.
The Bodoland militants have been fighting for a separate state for indigenous Bodo people who reside in Darjeeling area and Assam. Last month, the Centre's forces had started an offensive against the rebels in their remote hideouts.Prime Minister Narendra Modi on Tuesday termed the attack as "an act of cowardice".
"Killing of innocent people in Sonitpur and Kokrajhar is an act of cowardice", Modi tweeted. He said "our thoughts and prayers are with the families of the victims of the deceased." Modi said he has spoken to both Union Home Minister Rajnath Singh and Assam Chief Minister Tarun Gogoi and Singh will travel to the state to take stock of the situation
(With PTI inputs)