Assam Police said there were no reports of any casualties in the explosions.
Four explosions - three in Dibrugarh and one in Charaideo districts - took place in the Upper Assam in a span of 10 minutes on Sunday morning, as the country celebrated its 71st Republic Day.
There were no reports of any deaths as most people were mostly indoors since it was a holiday, a police officer said. All four blasts took place between 8:15 and 8:25 am, he added.
The first blast took place at Graham Bazar, near NH37 near a garbage dumping area. The second near back side of the boundary wall of Sikh National School at Marwari Patty area of Dibrugarh, the home district of Assam chief minister Sarbananda Sonowal.
Another grenade blast was reported in Duliajan in the same district.
The fourth blast was reported at Sonari in Charaideo district.
A massive operation has been launched and the state government has ordered a probe into the incidents.
There is report of yet another blast Doomdooma in Tinsukia districts, but police is yet to confirm this.
According to sources, the explosions were suspected to be carried out by the ULFA (Independent) militant group.
The ULFA (I), along with several other such outfits in the Northeast, had called for a boycott of Republic Day celebrations.
"This serial blasts are a cowardly act of the militants and we strongly condemn it. Today the people of Assam, in all 33 districts have took part in Republic day celebrations showing faith in this government and its developmental agenda. We have ordered a probe into the blasts and to take strict action," said Assam Chief Minister Sarbananda Sonowal.
Assam police suspects ULFA (I) led by Paresh Barua to be behind the attack.
"We have launched counter operations and have got some crucial leads. We suspects that the ULFA (I) is behind these blasts. They are frustrated with decline in insurgency and wanted to make their presence felt" said Assam DGP Bhaskar Jyoti Mahanta.
A source in the chief minister's office said Mr Sonowal has asked Assam Director General of Police Bhaskar Jyoti Mahanta to deal with the situation and catch those involved in the explosions at the earliest.