Conveying her deepest condolences to the families of the bereaved, Gandhi urged upon the authorities to provide immediate medical assistance to the injured.
She also asked the Uttar Pradesh Congress Committee and party workers to provide all help in relief works and possible assistance to the families of the injured, the party said in a statement.
"Congress president Sonia Gandhi expressed shock and horror at the terrible tragedy in Raebareli," it said.
An explosion ripped a boiler in the state-run power giant NTPC's Unchahar plant in Raebareli killing at least 12 people and injuring 90-100 others, according to Uttar Pradesh Principal Secretary (Home) Arvind Kumar.
Congress vice president Rahul Gandhi said he was disturbed by the incident and urged the authorities to provide all help to the injured.
"Disturbed by the NTPC plant incident. My condolences are with the families of those who died. Urge the authorities to provide immediate help to the injured," he said in a tweet in Hindi.