A constitutional challenge begins in an Ontario courtroom Monday that could affect families across the country.

Some children's rights advocates are trying to have a law struck down that makes spanking a child legal.

Lawyers spent the weekend finishing their arguments against Section 43 of the Criminal Code of Canada, which states that parents can use reasonable force to correct a child.

Critics of the law call it a legal defence for child abuse.

"We can point to many cases ... in which children have been injured, implements or weapons have been used ... and the parent or teacher charged in the assault was acquitted on the basis of Section 43," according to Cheryl Milne, a lawyer with Justice for Children and Youth.

But some parents point to surveys that show 70 per cent of Canadian couples have spanked their children at some point, and feel the punishment is appropriate.

"I think it's important that we support those parents, and we want to uphold parental authority," says Terry Ruddell of the Canada Family Action Coalition.

Ottawa agrees. The Minister of Justice says federal lawyers will be in court this week arguing that Section 43 is constitutional.

But some lawyers say the federal government is being hypocritical.

"The government has stated elsewhere that hitting children is a bad idea, and it's wrong," according to Paul Schabas of the Canadian Foundation for Children, Youth, and the Law.

"So in that sense we'll be pointing out to the court the hypocrisy of the government's position," Schabas adds.

Regardless of what the Ontario Superior Court decides, lawyers predict the case will eventually wind up before the Supreme Court of Canada.