Farhan Ahmed was arrested yesterday and his mobile phone seized. "He had taken the SIM by furnishing a fake address," Additional Superintendent of Police Chiranjeevi Sinha said.
Ahmed, 20, was arrested near a barrage under Tarkulwan police station area, he said.
He is being interrogated by local police officials and intelligence officials from Delhi, Mr Sinha added.
Ahmed had allegedly told the ADG (Law and Order) on his mobile phone that he would blow the UP legislature building on Independence Day.
Deoria is 325 km from state capital Lucknow and the district is a part of Gorakhpur division, the native place of Chief Minister Yogi Adityanath.
Ahmed is a resident of Kawlachhapar village under police station Rampur Karkhana.
In a major security breach, a dangerous plastic explosive was found in the Uttar Pradesh Assembly, which today unanimously recommended an NIA probe.