Four teenage boys have been arrested on suspicion of murdering a 12-year-old girl in Liverpool city centre.
Police said Ava White suffered "catastrophic injuries" after an argument escalated into an assault at around 8.39pm on Thursday, shortly after the Christmas lights switch-on took place.
It's believed a knife was used but it is yet to be recovered, said Detective Superintendent Sue Coombes.
One of the four boys arrested is aged 13, two are 14 and one is 15. They were detained at different locations.
When police arrived they found Ava - who was with friends - collapsed and receiving first aid from a member of the public.
She was taken to Alder Hey Children's Hospital by paramedics, but died a short time later.
A post-mortem examination is still to be carried out.
Assistant Chief Constable Jon Roy said Ava's family is being supported by specialist family liaison officers.
"We believe that Ava and her friends had been involved in a verbal argument which culminated in Ava being assaulted causing catastrophic injuries," he said.
"And we understand that the offenders were then seen to run up School Lane across Hanover Street and on to Fleet Street.
"We have arrested four males, one aged 13 years, two aged 14 years and one aged 15 from Toxteth, on suspicion of murder and they are currently being interviewed at police stations on Merseyside.
"Liverpool City Centre was very busy at the time of the incident as the lights on the Christmas tree on Church Street had been officially switched on a short while earlier.
"And we would appeal to anyone who was on Church Street who saw the incident, or may even have captured it on their mobile phone, to come forward."
Ava went to Notre Dame Catholic College and its head teacher, Peter Duffy, called her "an incredibly popular girl with a fantastic group of friends".
Rebecca Flynn, head of Ava's former school, Trinity RC Primary, said: "Our prayers and thoughts are with Ava's family and her friends.
"We are left with lasting memories of a bright and respectful little girl."
Police ask anyone who witnessed the incident or filmed it to contact them either on Twitter at MerPolCC, via Merseyside Police Contact Centre on Facebook, or by calling Crimestoppers on 0800 555 111 with reference 0899 of 25 November.