Not too many details yet, but the coincidental timing to the Boston Marathon bombings make this even more of an eyebrow-raiser:

Canadian police and intelligence agencies will announce later today they have thwarted a plot to carry out a major terrorist attack, arresting suspects in Ontario and Quebec, CBC News has learned. Highly placed sources tell CBC News the alleged plotters have been under surveillance for more than a year in Quebec and southern Ontario. The investigation was part of a cross-border operation involving Canadian law enforcement agencies, the FBI and the U.S. Department of Homeland Security. The arrests Monday morning were co-ordinated and executed by a special joint task force of RCMP and CSIS anti-terrorism units, combined with provincial and municipal police forces in Ontario and Quebec.

Authorities tell CBC News that this has no connection to the Boston attack. No motive or target(s) have yet been released, but the CBC is already referring to the Toronto 18 case from 2006, an Islamist plot to conduct a major terrorist attack in Canada.

Stand by for more details as they emerge.

Update: The RCMP has two people under arrest in a “bomb plot,” according to the Globe and Mail, but the report also notes this toward the end:

Several Muslim community leaders were called by Mounties early Monday to drop what they were doing and come at 2:30 p.m. to the RCMP police station near Toronto’s Pearson International Airport.

The G&M also reports that their sources indicate a smaller conspiracy than the Toronto 18.

Update: The RCMP will hold a presser at 3:15 ET.

Update: This explains why the FBI was involved:

A potential bomb plot on a passenger train over the Niagara River has been averted, a police source says. … One police source said there were several scenarios being looked at during the year the suspects were tracked, including the train plot. One of the plots uncovered by the RCMP and other agencies was a planned bombing of a passenger train on the bridge that connects Canada and the United States at Niagara Falls, a police source says. “The plan was to take out a train with passengers on board and the crossing trestle,” said a source. “It was meant to be spectacular and there would have been a lot of carnage.”

That sounds like al-Qaeda’s MO — attacking transportation targets for maximum visibility. We’ll see whether that’s the case when Canada holds its presser.

Update: I was correct about it being an al-Qaeda plot:

RCMP: 2 men arrested were allegedly part of an al Qaeda-supported plot to attack a passenger train in #Canada. on.cnn.com/17SHjCq — CNN Breaking News (@cnnbrk) April 22, 2013

Canadian Police: Believe plot against passenger train was supported by Al Qaeda – from broadcast — Breaking News (@BreakingNews) April 22, 2013

Update: Canada names the two terrorist suspects now in custody:

Canadian police and intelligence agencies have made two arrests in connection with a planned major terrorist attack to derail a Via Rail passenger train in the Greater Toronto Area. Two suspects are in custody, one arrested in Toronto and one in Montreal, following an RCMP counter-terrorism investigation. At an afternoon news conference, the RCMP named the suspects as Chiheb Esseghaier, 30, and Raed Jaser, 35, neither of whom are Canadian citizens, and said they were linked to al-Qaeda. RCMP Assistant Commissioner James Malizia said that while the suspects had the capacity and intent to carry out an attack, there was no imminent threat to the general public, rail employees, train passengers or infrastructure.

Search warrants are being executed now.