A police officer who was captured on video using an "open hand strike" on 14-year-old girl used "reasonable force" to restrain her, Lancashire Constabulary has said.
Footage posted online shows the male officer and a female colleague struggling with the girl in Accrington Market.
Lancashire police said the open hand strike was used when the girl "attempted to grab the officer's taser and radio".
In the video, a female officer is also seen trying to restrain the girl as bystanders look on and barrack the officers.
One witness is heard saying: "This man is smacking up this little girl."
People shout "get off her" as someone comments: "Watch these two (officers) lose their jobs after this."
At the end of the footage the girl appears to be on the ground. By that stage, more officers have arrived.
The video has been viewed more than two and a half million times on Facebook.
Following the scuffle, the girl and a 52-year-old woman were arrested on suspicion of assaulting a police officer, and two officers were taken to hospital for treatment to a cut and a bite wound.
The girl has been referred to the youth offending team after being interviewed, while the woman was given a police caution.
"I am aware of the mobile phone footage of this incident and have reviewed it personally," said Chief Constable Andy Rhodes.
"My officers have responded as a result of a concerned member of the public and have been met by a very hostile situation.
"As they've brought the situation under control officers have been assaulted and have received medical treatment as a result."
Chief Constable Rhodes added: "It is very easy to judge from the side lines without the full facts when you've never been in a similar situation yourself.
"These are your police officers and they are here to keep you safe.
"They do not come to work to be assaulted and they have my full support.
"Perhaps those filming the incident may have better used their time helping the officers to calm the situation."
A statement from Lancashire Constabulary said the officer used "reasonable force to bring the situation under control".
The incident has divided people on Facebook. One person reacting to the video said: "I respect police but this is bang out of order."
Another said: "Give that officer a pay rise."