A three-year-old boy was seriously injured in a suspected acid attack.
West Mercia Police believe the child was deliberately targeted in Home Bargains on Shrub Hill Retail Park at Tallow Hill.
They have released images of three men they would like to speak to.
The boy was taken to hospital where he is being treated for serious burns to his arm and face.
A 39-year-old man from Wolverhampton has been arrested on suspicion of conspiracy to commit grievous bodily harm and remains in police custody.
Chief Superintendent Mark Travis said: "At this time we are treating this as a deliberate attack on a three-year-old boy.
"The incident will rightly shock the local community and I would like to reassure local people that we are carrying out a thorough investigation to identify those responsible.
"At this time, the motive for the attack is unclear. Officers are working hard to establish exactly what happened and what the substance involved was.
"The three men in the photo may have information vital to our investigation. I am sure someone will recognise them and I would urge anyone with information to contact us as a matter of urgency."
The deputy leader of Worcester City Council, Adrian Gregson, said he was in the shop next to Home Bargains shortly after the attack.
He told the Press Association: "I was in the shop next door when it was going on, it was around mid-afternoon.
"I didn't know what the situation was - there were a couple of fire engines and a fire incident vehicle and a police car. By that point the ambulance had been and gone.
"It is the first time, as far as I'm aware, this has happened in Worcester."
Robin Walker, MP for Worcester, described the incident as "horrific" and urged anyone with information to contact the police.
"The shock will be universal, anyone conducting such an attack on a small child is just appalling," he added.
"It's an issue which rightly there has been a lot of concern in Parliament and a move to toughen sentences for any kind of acid attack.
"But particularly the nature of this being a small child is very concerning indeed.
"At this stage the key thing is for the police to act quickly and see if they can track down the perpetrator and understand what's behind it because it is an unthinkable thing to happen."
Worcester City Council leader Marc Bayliss said the attack was "absolutely pure evil" and added: "Worcester is not that sort of place. We are a quintessential small English city.
"I have never heard of an acid attack in Worcester so this is absolutely not something we have any experience of.
"We are a very safe city, we are one of the safest places in England according to statistics.
"If anybody knows anything about this I would urge them to come forward - think if this was a member of your family, an innocent child, a three-year-old probably scarred or damaged for life by this.
"We need to bring the perpetrators to justice, and quickly."
A local shop worker, who did not want to be named, said he saw a number of emergency services outside the store on Saturday.
"There were a couple of fire engines and a couple of police as well.
"Customers were still going in and then they completely shut up at about 3pm.
"It's shut now. There are two officers outside saying that it's shut."
A spokesman for the West Midlands Ambulance Service said it was called to Home Bargains at 2.24pm on Saturday "to reports of a young boy who had suffered a severe reaction to an unknown substance".
"An ambulance was sent and arrived in eight minutes. The crew assessed the boy at the scene and liaised with the trust's hazardous area response team for specialist advice.
"After treatment, he was taken to hospital in Worcester. We have no update on his condition."
A Home Bargains spokeswoman said: "Our thoughts are with this young child and his family at this very difficult time."
The three-year-old boy injured in a suspected acid attack in Worcester on Saturday has been discharged from hospital, Ch Supt Travis told Sky News.
He said the long-term implications of the injuries were uncertain, but he hoped they would not be life-changing.