Merseyside Police said the girl, named as Ava White, had been with friends at the time. The lights on the Christmas tree on Church Street, at the heart of the city's main shopping area, had been switched on shortly beforehand.
Four male suspects, one aged 13, two aged 14 and one aged 15 years, all from the city's Toxteth neighborhood, have been arrested on suspicion of murder.
Officers were called to reports of an assault at about 8.40 p.m. local time, according to a police statement released Friday. They found Ava collapsed on the ground, with a member of the public giving first aid.
"Paramedics attended at the scene and Ava was taken to Alder Hey Children's Hospital, where sadly she died a short time later despite the best efforts of medical staff," the statement said.
Assistant Chief Constable Jon Roy said the Merseyside force's thoughts were with Ava's family.
"Their world has been torn apart and no parent should ever have to face that knock on the door from police officers to say that their child has died," he said.
"We believe that Ava and her friends had been involved in a verbal argument which culminated in Ava being assaulted causing catastrophic injuries."
Roy appealed for anyone who may have seen the incident or even captured it on mobile phone footage to come forward to police. The city center was busy at the time because of the official switching on of the lights, he said.
A number of streets remain cordoned off as forensic officers investigate, police said. Additional officers will be out in the city center in the coming days to provide reassurance to members of the public.
"Ava's death should be a reminder to us all about the part that we each have to play in eliminating violence against women and girls," said Roy.
She died on White Ribbon Day, a global campaign to end violence against women, he added. Earlier that same day, partners from across the city had come together to mark the start of 16 days of activities intended to raise awareness of the issue and show that violence against women and girls would not be tolerated, he said.
Steve Rotheram, metro mayor of the Liverpool City Region, said he was horrified by Ava's death, adding that it was appalling such an attack had taken place on one of the busiest streets in the area on one of the busiest days of the year.
"I am angry today for Ava and her family, for the parents across the region worrying about their children's safety, and angry that such a heinous act should take place here," he tweeted.
Peter Duffy, headteacher of Notre Dame Catholic College, which Ava attended, said she "was an incredibly popular girl with a fantastic group of friends," according to Britain's PA news agency.
"Our deepest thoughts and prayers go out to Ava's family and friends and all those affected by this utterly tragic event," he is quoted as saying. "My staff are working with students to provide all the support they need at this traumatic time."