Twelve people have been arrested after drug officers busted a ring suspected of distributing fake pills laced with fentanyl, police say.

Seven men and five women were charged after police executed search warrants at various locations around the city — including a single family home, two townhouses, two apartments and a storage locker — the Ottawa Police Service said Thursday.

Officers seized fentanyl powder, cocaine, methamphetamine and counterfeit pills believed to be laced with fentanyl. The pills will undergo an analysis from Health Canada to confirm the presence of the powerful opioid, police said.

The fentanyl seizure is the largest in Ottawa police history, said Staff Sgt. Rick Carey of the force's drug unit.

'Mass casualties'

"We've seized thousands of pills today. So the potential there [was] for mass casualties," Carey told CBC News.

"We're trying to make sure this doesn't hit the street. Because who's it being accessed by? Youth that aren't aware as to what's going on."

Drug officers also seized a cache of weapons that included three handguns, two assault rifles, two regular rifles, two stun guns disguised as cell phones, three sets of brass weapons and a shotgun. 

rick carey steve bell ottawa police fentanyl arrests Feb. 16 2017

Staff Sgt. Rick Carey, left, and Deputy Chief Steve Bell take questions about what Ottawa Police Are calling its largest fentanyl seizure ever on Feb. 16, 2017. (CBC)

The people arrested will be charged with possessing the proceeds of crime, possession for the purposes of trafficking and various weapons-related offences, said police.

They are expected to be charged today and will appear in court Friday, police said. Their names have not been released.

14 fentanyl deaths in 2015

One of the locations where police carried out the arrests Thursday was at a home at 451 Ancaster Ave. in west Ottawa.

A woman was being led away in handcuffs when CBC News arrived on the scene.

The arrests come after an investigation launched by the police force's drug unit in September 2016 into the suspected drug distribution network, police said.

451 Ancaster Avenue drug fentanyl raid police Feb. 16, 2017

This house at 451 Ancaster Ave. was one of the locations where Ottawa police made arrests on Feb. 16, 2017, as they busted a suspected fake drug distribution ring. (Giacomo Panico/CBC)

Officers believed the ring was lacing counterfeit pills with illicit fentanyl, a potent opioid that has swept the country and caused 14 overdose deaths in Ottawa in 2015.

The Ontario Provincial Police were involved in the investigation as the pills were believed to have been sold in rural eastern Ontario as well, Ottawa police said. In addition to the drugs and weapons, officers seized more than $130,000 in cash.

Earlier this week, police and public health officials issued a warning that there had been a string of near-fatal overdoses believed to have been caused by fake Oxycontin and Percocet pills laced with fentanyl.

"We're seeing more and more serious injuries and deaths relating to those sorts of pills," said Deputy Chief Steve Bell.

"We're trying to prepare ourselves and our community to fight it."

guns rifles handguns brass knuckles weapons seized feb 16 2017

In addition to the drugs, police also seized handguns, rifles, assault weapons and brass knuckles in the Feb. 16, 2017 raid. (Ottawa Police Service)