NEWBURYPORT - Police say the dog-sniffing method used to identify pot-toting teenagers as they walked around downtown last weekend is accurate and effective. But civil rights advocates say it is not only a violation of individual rights, it may be illegal.

It would have been illegal for police to randomly search every person walking downtown Saturday night. The Fourth Amendment's protection against unreasonable search and seizure requires police to have probable cause before conducting a search.

On Saturday, Newburyport police brought in a K-9 unit from the Essex County Sheriff's Department. Working with a labrador retriever named Mackie, who is trained to alert his handler when he smells illegal drugs, police identified teens carrying marijuana without having to physically search them. With the probable cause provided by Mackie's tip-off, police said they were justified in executing physical searches.

But the civil rights groups maintain the legal precedents cited by police primarily pertain to using dogs to find drugs in luggage and vehicles, not on someone's person.

Sending the dog along through the crowd sniffing for drugs does not constitute a search, said Lt. Rick Siemasko of the Newburyport Police, who is also a lawyer. "The case law is very clear on this," Siemasko said. "Using a trained drug-sniffing dog is not a search that you're protected from by the Fourth Amendment."

Using dogs in this kind of drug enforcment action isn't uncommon, Siemasko said. Newburyport did it last summer, and the Sherrif's Department regularly lends drug-sniffing dogs to local communities for similar actions.

However John Reinstein, the legal director of the American Civil Liberties Union of Massachusetts, said that while using drug-sniffing dogs to search vehicles without any other cause is legal, using dogs on a person is not the same thing.

"If it's a search, it's a search without probable cause," he said. "I think there's a substantial question about the legality of what the Newburyport Police are doing."

In 1983, the U.S. Supreme Court ruled in United States v. Place that the canine sniff is a unique category, and that the use of a dog to detect narcotics in someone's luggage "did not constitute a 'search' within the meaning of the Fourth Amendment."

In the court's 2005 decision on the case of Illinois v. Caballes, it ruled that the use of drug-sniffing dogs to detect drugs in automobiles on routine traffic stops is permitted by the Fourth Amendment.

Massachusetts laws frequently give residents greater protection than federal law, but a few months after the Caballes decision, the Supreme Judicial Court of Massachusetts issued a similar ruling in Commonwealth v. Feyenord, noting that it is unreasonable to expect privacy about the odor of drugs emanating from one's car.

Reinstein said those precedents aren't relevant, because using dogs to sniff drugs in a vehicle and using dogs to sniff drugs on someone's person aren't the same thing.

"The law recognizes a substantial difference between the two," Reinstein said.

Bill Downing, a director of the Massachusetts Cannabis Reform Coalition, a local affiliate of the National Organization for the Reform of Marijuana Laws, said he is disturbed by the sweeps.

"This sounds like a violation of people's rights, in spirit if not in law," Downing said. "This is the problem with marijuana crimes: The only way police can enforce these laws is by doing sneaky things. It makes the police look bad, and it's not good for their relationship with the community."

Siemasko doesn't buy the argument that police should confine their Inn Street enforcement to crimes that directly affect the public.

"So as long as the drug dealers are well-behaved, they're allowed to deal drugs?" Siemasko asked. "That doesn't make sense."

However, none of the arrests made on Inn Street with the help of the drug dog has led to charges of selling drugs.

Siemasko said that one person arrested last summer had several bags each containing a small amount of marijuana, suggesting that that person may have been dealing.

Whether any of the people arrested on Inn Street over the weekend will challenge their detainment remains to be seen, but Steven Epstein, a director of the Massachusetts Cannabis Reform Coalition and a defense lawyer specializing in search and seizure cases, said it is unlikely.

"Far too few people challenge the grounds on which they were arrested in this sort of situation," he said.