"The blast took place at around 7:55 pm near the Paltan Bazar police station, which is next to the railway station. Six persons have been injured, two of whom are police home guards," Kamrup (Metro) Deputy Commissioner Ashutosh Agnihotri told reporters.
The grenade was hurled when police personnel were checking vehicles at GS Road.
Of the six injured, the condition of two are stated to be serious, he said.
"We have to investigate who did this, but the inputs say that the suspect is ULFA," Agnihotri added.
ULFA militants normally try to create trouble during the run-up to the Independence Day.