Parents in England and Wales who smack children so hard it leaves a mark will face up to five years in jail under new laws in force from Saturday.
'Reasonable chastisement' is legal
Mild smacking is allowed under a "reasonable chastisement" defence against common assault.
But any punishment which causes visible bruising, grazes, scratches, minor swellings or cuts can face action.
The law is flawed and there should be a total ban on smacking, according to child protection charity the NSPCC.
It said the new law will only confuse parents and leave children still at risk of abuse.
NSPCC boss Mary Marsh said: "There is a risk parents may choose to hit children on parts of their body where injury is less visible, such as the head, which can cause serious harm.
"Defining acceptable ways to hit children should become a thing of the past.
"It should be just as wrong to hit a child as it is to hit an adult."
The measures were passed in the Children Act last November, when the government suffered a rebellion by 47 Labour MPs who wanted a total ban on smacking.
A similar law is already in operation in Scotland.
A spokeswoman for the Department for Education and Skills said the new laws were a "common sense" decision to balance child protection with parents' autonomy.
She added: "Abuse is abuse and should be prosecuted. We are not going to tolerate criminal violence against children."
Supporters of smacking say it is an essential part of parents being able to discipline their children.
The head teacher of the Christian Fellowship School in Liverpool, Phil Williamson, said the new rules would be impossible to police.
"The law doesn't say how big the mark has to be, how long the mark has to last for.
"It also ignores totally black children who don't mark and don't go red. It's just a ridiculous law," he said.