Marak had been admitted to the hospital for gall bladder
operation around a week back. After getting information of his stay here along with his wife, Assam Police detained him on Saturday night, Assam's ADGP (Law and Order) A P Rout said.
"We informed the Meghalaya police accordingly. They reached here yesterday and took him to Meghalaya," Rout said.
Marak was involved in many cases of murder, kidnap and extortion, and was wanted by the police of both the states.
UALA is a breakaway faction of Achik National Volunteers' Council (ANVC) and is fighting for separate Garoland state.