The law, which came into force on January 1, was intended to tackle the appalling conditions in which some unlicensed breeders kept their animals.
It allows only breeders licenced by national professional organizations to breed dogs and cats for commercial purposes.
And in another measure designed to shut out casual, unregulated trade, it also prohibits the purchase and sale of pets at outdoor markets.
"Animals from illegal breeders have a horrible life, worse than stray dogs," said Izabela Dzialak, director of a shelter in Celestynow, near Warsaw.
"They are often starving and crammed in cages filled with their own excrement," she added.
Volunteers at animal shelters are already struggling to care for the strays and dogs abandoned by breeders after the new legislation came into effect.
But Dzialak saw that as a temporary headache.
"We might have a hard time with the large number of dogs coming in from illegal breeders for now, but in two years the problem will be solved," she said.
Already however, some breeders are trying to find ways around the ban.
"Selling a leash for 400 zlotys (95 euros, $129), will throw in a German Shepherd puppy for free" says one ad among thousands posted on popular Polish Internet shopping sites.
For Tomasz Justyniarski, spokesman for Poland's Guard (SdZ) animal rights association, the intention is clear.
"It's an obvious attempt to circumvent the new law banning all trade and husbandry of animals without a licence," he said.
"Since January, we receive one alert on average per day on illegal breeders in the Warsaw area," selling over the Internet, said Justyniarski.
When animals were seized from illegal breeders, the SdZ group did its best to find them new homes, he added.
But as long as there is money to be made in the trade, the less scrupulous breeders will continue, say animal welfare groups.
"The breeding and trade of unregistered pets is a widespread phenomenon in Poland and it isn't subject to any tax regulations," said Cezary Wyszynski, of the "Viva!" animal rights foundation.
In addition, many illegal breeders in Poland targeted the lucrative German and Austrian markets: over there, dogs can fetch a higher price, he explained.
One illegal breeder in Warsaw, who spoke on condition of anonymity, made it clear he would not let the new crackdown stop him.
"I've been selling dogs for five years. The income supplements my pension," he told AFP.
He kept two dogs, a German shepherd and a Saint Bernard, at his house in a Warsaw suburb, earning about 14,000 zlotys (3,300 euros, $4,507) each year from the sales of their puppies, he said.
"The new law will certainly complicate my life, but I won't stop so long as I need the money," he insisted.
And, he added: "There are many others like me."
If he did have a licence, he would be able to ask, on average, 60 percent more than the current price at which he was selling the puppies, he said.
But all the paperwork -- and the strict rules about breeding -- made it more trouble than it was worth.
Grzegorz Kurkowski, chief state veterinarian in Otwock near Warsaw, welcomed the new law but two additional measures were needed, he said.
"We still need to develop a coherent system of electronic tagging for animals and a sterilisation programme," he argued.
"This would solve the problem permanently."