The child suffered serious burns to his arms and face after being targeted while sitting in a pushchair at Home Bargains in Shrub Hill Retail Park.
He was treated in hospital before being discharged. West Mercia Police have released images of three men they would like to speak to about the incident.
Chief Superintendent Mark Travis said the substance was either thrown or sprayed towards the child while the young family were out shopping at around 2.15pm on Saturday.
He said in a statement: “At this time we are treating this as a deliberate attack on a three-year-old boy. The motive for the attack is unclear. Officers are working hard to establish exactly what happened and what the substance involved was.
“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.”
He added: “We are uncertain about the long-term implications of those burns, but we do hope that these won’t be life-changing injuries.”
He said that the three men pictured in the images from the store were “in very close proximity to the little boy” and ”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,” added Chief Supt Travis.
A 39-year-old man from Wolverhampton has been arrested on suspicion of conspiracy to commit grievous bodily harm and remains in police custody.
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 said.
“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.”
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,” a spokesperson said. ”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.”
Chief Supt Travis told Sky News on Sunday afternoon the boy had been discharged from hospital but said the long-term implications of the injuries were uncertain.
The Home Bargains store was closed shortly afterwards and remained shut on Sunday as the police investigation continued.
West Mercia Police said anyone with information should call 999 or the independent charity Crimestoppers on 0800 555 111.
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