Police in the Indian state of West Bengal say they have arrested a top separatist leader and three accomplices from the neighbouring state of Assam.
Ulfa has been fighting Indian forces for more than two decades
West Bengal police said a special team arrested Mrinal Hazarika, a senior leader of the United Liberation Front of Assam (Ulfa), early on Thursday.
Mr Hazarika was detained at a hotel in the town of Siliguri, police say.
Assam's police intelligence chief told the BBC that Mr Hazarika held the rank of a self-styled Ulfa "major".
Khagen Sarma described the Ulfa leader as a "major catch" and said he was being questioned by the West Bengal police.
He said that "at some point" he may be handed over to police in Assam.
Mr Hazarika is the second top leader to be caught by the security forces in the past month.
In April, Ulfa's western zone commander, Lal Deka, was killed in a clash with the army.
Police said Mr Hazarika had replaced Mr Deka and was visiting Siliguri to raise funds for the underground organisation.
Ulfa is currently holding peace negotiations with the Indian government through a committee of civil society leaders called the People's Consultative Group (PCG).
The PCG has twice met Indian government officials but Ulfa has yet to announce a formal ceasefire, although it says that it wants an end to military operations in Assam.
Assam Chief Minister Tarun Gogoi, after being sworn in again for a second term this week, called on Ulfa leaders to announce a formal ceasefire and join "direct talks" with the government.
But the Ulfa leadership is upset over what it says is the continuing police and military operation against its fighters, and says that the atmosphere for talks has been spoiled.
Thousands of people have died since Ulfa began fighting for independence in 1979.