Authorities said violence spread to newer areas in the hill district. An indefinite curfew clamped in Diphu town also continued Thursday.
One man injured in violence in Diphu Wednesday died in a hospital at night, police said.
On Thursday, protesters defied the curfew to pour out of their homes and attacked government offices by attempting to set them on fire.
These included the All India Radio station and a branch office of the Asom Sahitya Sabha, the apex literary body of Assam.
Locals said the violence spread to Dilai area near Diphu town. Hundreds, including women, attacked some government offices.
Some damaged portions of rail tracks between Diphu and Doldoli, leading to police firing. No one was injured in the incident.
Deputy Inspector General P Saloi said there was adequate security deployment in the affected areas.
"We are trying our best to contain the situation. More forces are on the way to assist the police in tackling the situation," he said.
Violent protests rocked Karbi Anglong district Wednesday morning, leading to killing of one person and injury to 19 others and forcing the administration to impose indefinite curfew in Diphu.
The protesters had Wednesday attacked the houses of Lok Sabh MP from Diphu Biren Sing Ingty and legislator Bidya Sing Engleng.
Members of several student bodies and social organisations in Diphu took to the streets Wednesday demanding a separate state on the lines of Telengana.