The attack took place in Diphu town, about 270 km south of Guwahati, the state's main city.
The rebels earlier killed 48 people, mostly labourers and traders, in a series of coordinated overnight attacks.
Police said heavily armed United Liberation Front of Asom (ULFA) guerrillas gunned down at least 32 people in the state's eastern district of Tinsukia in separate raids.
A further 15 labourers, including a woman, were killed in two strikes in adjacent Dibrugarh district. Another person was killed when militants triggered a blast in Sivasagar district.
"The death toll now stands at 48," said Tarun Gogoi, Assam's chief minister.
Police said the violence was an attempt to intimidate people after an independent opinion poll by a peace group in nine districts of the oil and tea producing state showed 90 per cent of the people rejected the ULFA's separatist demands.
Authorities clamped a round-the-clock curfew in Tinsukia after thousands of angry residents came out on the streets protesting the failure of the security forces to provide safety.
Most of the victims were migrant labourers from the eastern state of Bihar who have been attacked by the rebels in the past to attract the attention of the federal government, security officials said.
No group has claimed responsibility for the attacks, but police and security analysts say the ULFA, which is fighting for an independent homeland for the Assamese people, was behind the killings.
The attacks came a day after Indian officials appealed to the ULFA not to disrupt national games next month which the rebels have threatened to disrupt with violence.
The insurgency in Assam has killed more than 20,000 people since it began in 1979.