Five children were among 11 people hurt when a stray firework shot into a crowd watching a Bonfire Night display.
The accident happened just minutes into a display which was being watched by some 15,000 people at Wicksteed Park near Kettering in Northamptonshire.
A spokesman for Kettering General Hospital said two children and an adult remain in hospital. One child may be transferred to a specialist burns unit.
The Health and Safety Executive has begun an inquiry into the accident.
Peter Tait, from St John Ambulance, said a number of children were involved because they were put at the front of the crowd.
"A Catherine wheel, nailed to a post when it started spinning, fell off and rolled into the barriers that divided the crowd away from the firework display," Mr Tait said.
He added that most of the injuries were burns to the lower limbs.
Dave Duke, 38, who had gone to the park with his wife and three children, said: "There was a massive explosion and then there was a thick, dark cloud of smoke.
'Something had gone wrong'
"We were running late and had stopped the car on the road half a mile away to watch the fireworks.
"A couple of rockets had gone up before there was this huge bang.
"I could then see fireworks exploding nearer the ground than you would expect. Then there was quiet. I said to my wife that I thought something had gone wrong."
The event - which was halted after the incident - was being staged by Kimbolton Fireworks, which was responsible for the display at the Trafalgar celebrations.
John Roberts, the managing director of the 147-acre park, said: "We are all shocked at this incident and our thoughts are with those who have been injured."
The accident has led to renewed calls for a tightening of fireworks' regulations.
But speaking on BBC Radio 5 Live, John Woodhead, the chair of the British Fireworks Association, warned a ban would make matters worse.
"The nuisance fireworks - the air-bombs, the bangers, the mini-rockets and all these things that were causing the problems - those have gone now, unless you buy them illegally.
"Banning will certainly not help, it'll make it worse."