The charges includes first-degree murder and conspiracy in the slaying of Salvatore Montagna, who was named by U.S. authorities as a former head of New York's notorious Bonanno crime family.
The six arrested include Raynald Desjardins, 59, a former confidant of Vito Rizzuto, the reputed head of the Montreal Mafia who is currently in a U.S. prison.
Desjardins, identified by authorities as a dominant figure in Montreal organized crime circles and in the construction industry, has a lengthy criminal record that includes a conviction on cocaine trafficking charges from 1993.
One of the few non-Italians within the Mob's inner circle, Desjardins had been the victim of an attempted murder last September by an assailant believed to be aboard a watercraft.
Tuesday's arrests came after nearly two years of carnage in Montreal's underworld, with brazen hit after brazen hit.
Montagna, known to police as a member of the Mafia, was gunned down and found near the home of Jack Simpson, 69, near Montreal on Nov. 24.
Police believe Montagna, whose nickname was "Sal the Iron Worker," was shot and ran to the L'Assomption River nearby to evade the gunmen. He was pulled from the water by emergency crews.
Police say Simpson was also tied to the Mafia. He was on parole at the time of the shooting but is currently incarcerated. He will face the same charges as the others.
Two of the others arrested are Vittorio Mirarchi, 34, described by police as another important player in the Mafia, and Felice Racaniello, 27.
Police said the men were all business associates — legitimate or otherwise.
Two others arrested without warrants are expected to face similar charges.
"We believe we have just dealt a major blow to organized crime, particularly Italian organized crime," Insp. Roberto Bergeron told a news conference.
Police wouldn't say if those arrested are linked to a rash of other kidnappings and slayings in the past two years.
"I presume that at their age, it wasn't their first offence," Bergeron said.
Montagna's slaying was one of the latest in a series of Mafia-related killings and disappearances amid an ongoing power struggle.
That struggle has intensified since Rizzuto was jailed in 2007.
Several killings and disappearances in late 2009 and early 2010 have robbed him of many of his closest family members and allies.
Rizzuto is due to be released in mid-2012.
Police declined to go into the specifics of the feuding.
"There have been numerous acts of violence which were undertaken on different factions throughout the Greater Montreal area and police agencies were on the trail of everyone," said Insp. Michel Forget.
"We can't tell you if they're linked with each other but there are many elements that led us to the arrests we've made today."
Montagna was born in Montreal but raised in Sicily and, although he moved to the United States at 15, he never obtained U.S. citizenship.
The married father of three was deported to Canada from the United States in 2009 because of a conviction for refusing to testify before a grand jury on illegal gambling.
He pleaded guilty to the minor charge, but it made him ineligible to stay in the U.S.
Montagna had no criminal record in Canada and re-entered this country without any trouble.
His arrival came mere months before members of the Rizzuto family began being killed.
Forget said more arrests involving the Mafia would likely be forthcoming.
"In general terms, our efforts are not done," Forget said. "What you're seeing here today is a major piece of the puzzle but we're still going to undertake many more actions in the near future."
The RCMP as well as municipal forces from Montreal, Longueuil and Laval were involved in the investigation.
Police executed 16 search warrants in connection with the arrests.
Arraignments are scheduled for Wednesday in Joliette, Que.