US singer Taylor Swift spoke of her horror on Tuesday at the news two children had been stabbed to death and six others were critically wounded after a ferocious attack at a dance event in northern England for young fans of her music.

A 17-year-old male is in police custody on suspicion of murder and attempted murder after the incident at the "Taylor Swift yoga and dance workshop" in Southport, north of Liverpool for children aged from six to 11 on Monday morning.

Two children were killed and nine others were injured, six critically, while two adults were also critically wounded, with police saying they believed had been trying to protect the young victims. Witnesses described what they saw as being like a scene from a horror film.

"The horror of yesterday's attack in Southport is washing over me continuously and I'm just completely in shock," Swift wrote on Instagram.

"These were just little kids at a dance class. I am at a complete loss for how to ever convey my sympathies to these families."

Her fans have raised more than 40,000 pounds ($51,440) online to help families of the victims and for the hospital where some of the children are being treated.

British Prime Minister Keir Starmer called the attack, which police are not treating as terrorism-related, "horrendous" while King Charles and his family expressed their shock at what had happened.

Merseyside Police said the motive was unclear but said they were not looking for anyone else in connection with the stabbings.

The incident shocked Southport, a quiet seaside town, and Home Secretary (interior minister) Yvette Cooper was one of many who paid sombre visits to the scene on Tuesday to lay flowers.