OTTAWA (Reuters) -Canadian Prime Minister Justin Trudeau plans to invoke rarely used special measures allowing him to tackle protests that have shut some border crossings and paralyzed downtown Ottawa, sources said on Monday.

       In the province of Alberta on Monday, police said they broke up an armed group that was prepared to use violence to back a blockade at a border crossing with the United States, authorities said.

       Trudeau plans to use the 1988 Emergencies Act, which allows the federal government to override the provinces and authorize special temporary measures to ensure security during national emergencies, the sources said.

       The act has only been used once in peacetime - by Trudeau’s father, former Prime Minister Pierre Trudeau - who invoked an earlier version of the act in 1970 after a small militant group of Quebec separatists kidnapped a provincial cabinet minister and a British diplomat.

       The sources declined to be identified, given the sensitivity of the situation. Separately, the prime minister's office said Trudeau would speak to reporters at 4:30 p.m. ET (2130 GMT).

       The "Freedom Convoy" protests, started by Canadian truckers opposing a vaccinate-or-quarantine mandate for cross-border drivers, have turned into a rallying point for people opposing the policies of Trudeau's government, covering everything from pandemic restrictions to a carbon tax.

       (Reporting by Steve Scherer and David Ljunggren;Additional reporting by Ismail Shakil in Bengaluru and Rod Nickel in Winnipeg;Editing by Paul Simao and Lisa Shumaker)