Manmohan Singh, the Indian prime minister from the ruling Congress party, was set to campaign in Assam on Tuesday.
It is the second time Gauhati has been attacked in a week and Singh said that the police had recently gained information that ULFA were planning an attack in the area.
On March 31, an explosion killed one person and injured 13 others.
The latest incidents occurred a day before the 30th anniversary of ULFA, which has previously carried out attacks around their anniversary and has been blamed for violence in Assam before.
ULFA is fighting for secession for the Assamese people in the northeast and is one of dozens of armed groups operating in the region.
They have accused the government of neglecting the Assamese while plundering the region's natural resources.
More than 10,000 people have died in fighting during the past 10 years.