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. 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. My Verdict
Hitting people should never be alright, whether it's a child or an adult. Hitting, whether it's disguised with the term spanking or not, is always violence. Violence is never the answer. That's my opinion.
Spanking, by its very definition, is violence. It's simply a milder form and a sometimes "controlled" form of violence, but it is still violence. I won't debate the ethics of it, but it's my belief that all violence is wrong. Most people wouldn't justify hitting an adult. A child should have the protection that an adult would have to live a life free of worry or the reality of being hit.
There are much more effective (and non-violent) ways to discipline a child.
People who falsely believe that need to read more parenting and child care books. They need to use their brains and not their hands as weapons. It's as insane as saying that we should regularly hit adults who don't do as they're told.
Just because something is legal does not make it right. Unfortunately, the law allows controlled violence to those under the age of 18. I think that it won't someday. As people and society has changed through the years, old commonplace injustices have been thrown out. I think this one will be, too.
“If we think of children as made in God’s image, I can’t think their being hit is part of God’s plan. We need to think and act respectfully towards children, for a gentler world now and in the future.” Rosemary Johnston Former Children’s Advocate, The United Reformed Church, 2006
"We call upon our governments to adopt legislation to prohibit all forms of violence against children, including corporal punishment, and to ensure the full rights of children, consistent with the Convention on the rights of the Child and other international and regional agreements".
Extract from 'A Religious Commitment to Confront Violence Endorsed at the 8th World Assembly of Religions for Peace Ktoto, Japan 2006.
