The plant in Unchahar is almost 30 years old and supplies electricity to nine states. The boiler pipe burst took place in the 500 MW power generating unit that started operating in March and was on trial. Reports suggest that the some workers sensed pressure building up in the boiler but it is not clear whether there was a delay in shutting the plant after the alarm or evacuating people to safety. Around 870 people work in the plant. The government is investigating whether there was any negligence.
A fire broke out and a huge ball of dust rose after the explosion.
"We were 80-90 metres away. The boiler started shaking and then there was a blast...gas exploded. Many fell unconscious," said a survivor.
In a statement, the NTPC has said that there was a "sudden abnormal sound...and there was an opening...from which hot gases and steam escaped affecting the people working around the area." Around 80 people were taken to hospital.
Hours into the rescue effort, there were fears that some workers might be trapped inside the unit.
"Rescue operations are still on," said senior official Sanjay Kumar Khatri on Thursday.
Congress leader Rahul Gandhi has interrupted his campaign tour of Gujarat for polls next month to visit the plant and the injured workers in Raebareli, which is his mother Sonia Gandhi's constituency.
Prime Minister Narendra Modi tweeted last evening to say that he was "deeply pained" by the accident. "My thoughts are with the bereaved families. May the injured recover quickly. The situation is being closely monitored and officials are ensuring normalcy is restored," he posted.