A police spokesperson said security forces early Monday arrested Prithvi Raj Rabha, a "most wanted bomber" of the ULFA in Assam's Goalpara district, about 130 km west of here.
"The arrest was made based on specific inputs that Rabha was planning to enter the area from adjoining Meghalaya to carry out some violent attacks," a police official said.
A large haul of explosives and ammunition was made from the arrested ULFA bomber.
In separate overnight raids, police recovered at least two improvised explosive devices in eastern Assam.
"There are definite reports of militants desperately trying to stage violent attacks ahead of Republic Day and we believe these explosives were part of their plan to attack," the official said.
Meanwhile, a massive security alert was sounded across Assam and other northeastern states with policemen, paramilitary troppers and soldiers deployed in vulnerable areas like oil installations, road and rail bridges, besides crowded marketplaces.
The intelligence reports come at a time when five separatist guerrilla groups in the northeast have called a 17-hour general strike to boycott Republic Day.
The militant groups that have called for a boycott of the Republic Day celebrations include the outlawed ULFA, the National Democratic Front of Bodoland (NDFB), the Kamatapur Liberation Organisation (KLO), fighting for a separate homeland comprising parts of Assam and West Bengal, the Manipur People's Liberation Front (MPLF), an umbrella group of several Manipuri rebel groups, and the Tripura People's Democratic Front (TPDF).
These groups are active in Assam, Manipur and Tripura.
As part of the boycott call Jan 26, the militant groups have called for a general strike from 1 a.m. to 6 p.m.
Militant groups in the northeast have for years boycotted national events to protest New Delhi's rule over the region.
On Sunday, militants blew up a railway track in Assam's Dhemaji district, while security forces gunned down two NDFB militants.
"We have mobilised troops in vulnerable areas to foil any militant attacks," a Manipur police official said.
Similar security alerts were sounded in Tripura and Meghalaya as well.