OTTAWA (REUTERS) - Canadian Prime Minister Justin Trudeau has accused protestors encamped in downtown Ottawa of engaging in acts that represented "a threat to our democracy" as demonstrators continued a three-week-old blockade at the centre of the capital.

Canadian officials have increasingly sharpened their tone, warning of extremist elements present among demonstrators who they say want to overthrow the government.  

Mr Trudeau invoked the little-used Emergencies Act on Monday, giving the government a wide range of additional temporary powers.

The Royal Canadian Mounted Police have sent in reinforcements and public broadcaster Radio-Canada said the province of Quebec was preparing to provide police.  

In a letter to the premiers of the 10 provinces, at least four of whom say there was no need to invoke the Act, Mr Trudeau said: "We are seeing activity that is a threat to our democracy and that is undermining the public’s trust in our institutions".  

Police in Ottawa had on Wednesday handed out flyers warning truck drivers and others paralyzing the downtown core of the city that they should leave or face arrest, but there was little sign of imminent action to move the nearly 400 vehicles out.

"We are going to take back the entirety of the downtown core and every occupied space," Interim Ottawa Police Chief Steve Bell said in a statement late on Wednesday.

Bad weather could potentially complicate any policing action. A steady rain fell on Ottawa early on Thursday and Environment Canada said this would later turn to snow, with up to 12 inches (30cm) expected by Friday morning.  

While the demonstrators initially protested against cross-border Covid-19 vaccine mandates for truckers and pandemic restrictions, they also have made clear their opposition to Mr Trudeau and some say they want to kick him out of office.  

Ms Tamara Lich, a prominent fundraiser for and organiser of the Ottawa protest, said in a video posted by CTV late on Wednesday: "I’m ready, I’m not afraid, and we’re going hold the line".  

Mr Trudeau is due to address the House of Commons on his decision to invoke the act at 10am Eastern Time (11pm Singapore time).  

Public Safety Minister Marco Mendicino said on Wednesday that a portion of the protesters "have strong ties to a far-right extreme organisation with leaders who are in Ottawa," referring to the arrest of 13 people in Alberta earlier this week linked to a border blockade there.  

Four have been charged with conspiracy to commit murder.