Thousands gathered near the prime minister's residence shouting "save our kids"; "we want our country back" and "stop the boats" as well as English football chants.
A large number of police wearing helmets formed a barrier around the protesters, who earlier threw flares and smoke canisters towards Downing Street.
Police were also braced for further trouble in the English seaside town of Southport, where Monday's stabbings took place.
More than 50 officers were injured on Tuesday night in violent clashes.
A 17-year-old male was in police custody on suspicion of murder and attempted murder after the bloody rampage on Monday at a "Taylor Swift yoga and dance workshop," a summer holiday event for children aged six to 11.
Thousands of people have gathered near the UK prime minister's residence shouting "save our kids". (AP PHOTO)
Disturbances in Southport on Tuesday took place after rumours spread on social media that the suspect behind Monday's stabbings was a radical Islamist migrant.
Police vans were set on fire and protesters hurled bricks, bottles and fireworks at officers and at cars parked at the town's mosque.
"Just to reassure the communities of Southport, who I am sure are really worried about 'are we going to see this again tonight?' - we are absolutely planning for this evening and for the weekend ahead," Serena Kennedy, chief constable of Merseyside Police, told reporters.
As well as killing three girls aged six to nine, Monday's attack in the normally quiet town in northwest England resulted in eight other children being stabbed.
Five remained in critical condition in hospital, along with two adults who tried to protect them.
Police are legally restricted in the details they can provide about the alleged teenage attacker but they have said the incident was not terrorism-related and that he was born in the UK, quashing speculation on his origins.
That did not prevent protesters, who police believe were supporters of an anti-Islam and anti-immigration group, from targeting the mosque and attacking police who tried to stop them.
Politicians and police said most of those taking part in the violence were not from the area, and that the clashes detracted from a large vigil attended by thousands to pay tribute to the young victims.
Police said 53 officers were hurt, with eight treated in hospital for serious injuries including fractures and head wounds.
Three police dogs were also hurt.
The police chief said she was disgusted and appalled by the violence.
Four men, aged between 31 and 39, have been arrested and police were seeking to identify others involved.
"They were there purely for hooliganism and thuggery in terms of bringing that level of violence and that behaviour on to the streets of Southport and that is not what this community is about," she said.
Prime Minister Keir Starmer said the "violent thugs from out town" would "feel the full force of the law".
On Wednesday, streets in Southport were littered with bricks from broken walls, smashed bottles, large rubbish bins and their contents.
Police vehicles set alight in the clashes a night earlier were visible.
"What I saw last night was absolutely appalling ... It was devastating and it's kind of taken away from actually what's gone on, which is the tragedy of those deaths," local resident David Burgess told Sky News.
Meanwhile, Taylor Swift fans have so far raised more than 270,000 pounds ($A533,400) to help families of the victims and for the hospital where some of the children were being treated.
