Dozens of charred bodies were discovered strewn in the remains of the tents, where people had gone to seek refuge from searing summer temperatures, and a state minister said the toll could be much higher.
"At least 100 people are dead," said Rajiv Sabarwal chief of police in the city of Meerut 80 kilometres (50 miles) north of New Delhi.
Yaqoob Qureshi, minister for minority welfare in the Uttar Pradesh state government told private TV networks: "There are reports that up to 200 people may have died, but we have no confirmation." Shoppers flocked to the three air-conditioned tents at the electronics fair to escape the heat outside but became trapped inside once the fire started, witnesses said.
Initial reports said a short circuit may have sparked the fire, which spread quickly through the three-storey-tall tents made of synthetic cloth with wooden floors, witnesses said.
Prime Minister Manmohan Singh expressed shock at the incident and conveyed his condolences to the relatives of those killed, his office said.
"So far, 52 bodies have been brought to the mortuary. More are still coming in," said C.B. Palliwal, a doctor at the city's government hospital.
He said more than 150 people, many with serious burns and other injuries, had been taken to hospitals.
"Almost 40 percent of the injured are badly burnt," he said, adding the hospital was not equipped to handle burn injuries. Many of the injured were being shifted to hospitals in New Delhi.
"Bodies are strewn around. Most of them are of women and children," eyewitness Rajesh Kumar told AFP.
Each steel-pole framed tent was about 100-metres (330-feet) long and had a one-way corridor with stalls on either side, making it almost impossible for people to escape once the fire began.
Banks of air-conditioners were set up to cool the tents, with the power units outside and the cooling units inside, witnesses said.
The tents each had capacity for more than 2,000 people. Witnesses said each was packed with people escaping 35-degree Celsius (95 Fahrenheit) heat on the last day of the five-day Brand India Fair at Victoria Park. Fire-fighters said the blaze, which started around 5:30 pm (1200 GMT), was brought under control as dusk fell.
Television footage showed blackened steel frames were all that remained. Plumes of thick black smoke billowed into the air as rescue workers pushed their way through onlookers to ferry casualties to hospitals and the dead to morgues.
Badly burnt bodies had been put in the back of trucks, police inspector Rakesh Tomar said. Rescue workers reported difficulty reaching the site because of fading light and large crowds of people who gathered to search for relatives.
Paramilitary troops, anti-riot forces and the army joined fire-fighters and police to tackle the massive fire. Confusion reigned as large crowds gathered around the devastated site. Chief Minister Mulayam Singh Yadav announced a payment of 200,000 rupees (4,500 dollars) to the relatives of each victim.
The fire was the worst such accident since 90 school children were killed in a fire that whipped through classrooms in the southern state of Tamil Nadu in July 2004.
In a similar tragedy, more than 250 Hindu devotees, mainly women and children, were crushed to death during a stampede after fire broke out and gas canisters exploded at a religious festival in western India in January 2005. Large crowds pose regular safety threats at events throughout India where at times millions of people mass under hazardous safety conditions.