The country's first-ever province-wide ban on pit bulls took effect Monday in Ontario, and breeders wasted no time in announcing a constitutional challenge to the ban.

Lawyer Clayton Ruby called the law "too vague" and "overly broad," and said the legislation lists a number of breeds that are "substantially similar" to pit bulls.

The ban makes it illegal to breed pit bulls in Ontario or bring the dogs into the province. Dogs already in Ontario are allowed to stay as long as they are sterilized, leashed and muzzled in public.

Registered purebreds are exempt from sterilization as long as they continue to participate in authorized dog shows.

Puppies born after Nov. 27, 2005 must be shipped out of the province, given to a research facility or destroyed.

Ontario breeders argue the ban violates breeders' constitutional rights. Hundreds of supporters of pit bull-type dogs gathered in front of the provincial legislature to protest on Sunday.

Other supporters, including the American Staffordshire Club of Canada, say that Ontario's law is so vague and unscientific that it will affect many animals that shouldn't be banned.

The law forbids Staffordshire bull terriers, American Staffordshire terriers, American pit bull terriers and any other dog with "an appearance and physical characteristics substantially similar to any of those dogs."

Last year, Ontario's attorney general Michael Bryant introduced the ban after a spate of vicious attacks on children, adults and pets by pit bulls.

In a widely-reported case, Toronto police fired more than a dozen bullets into two pit bulls that had turned on the man who was walking them as a favour for a friend. In another in London, Ont., a woman and her seven-year-old son watched in horror as a pit bull latched onto her husband's arm as he tried to keep the family puppy out of the dog's reach.

While some breeders are fighting the ban, others are leaving the province.

"It means I'm going to be moving out of Ontario," said Ann Mathews. Mathews has been breeding purebred American Staffordshire terriers for seven years. "It means I'll be leaving my three grandchildren, my five children and moving to a province that isn't so arrogant and isn't so stupid about the laws."