King Charles III stressed Canada's proud independence on Tuesday as he delivered a major speech to open parliament in Ottawa set against US President Donald Trump's repeated threats to take over the country.

"Democracy, pluralism, the rule of law, self-determination, and freedom are values which Canadians hold dear, and ones which the Government is determined to protect," Charles said, adding Canada was facing a "critical moment."

Canadian Prime Minister Mark Carney invited the 76-year-old British monarch – who is the Canadian head of state – to the capital, accompanied by Queen Camilla.

King Charles's "speech from the throne" was the first by a monarch in nearly half a century.

The king has never publicly commented on Trump's repeated talk of making Canada the 51st US state, but his language was closely watched for any reference.

Although the speech was read by the king as if his own words, it was in fact written by the prime minister's office to set out the government's priorities to "build Canada strong" and how it aims to achieve them.

Trump has also ripped up the world trade order and launched tariff wars against friends and foes alike, particularly targeting Canada, a member of the British Commonwealth.

"The system of open global trade that, while not perfect, has helped to deliver prosperity for Canadians for decades, is changing. Canada's relationships with partners are also changing," Charles said, in cautious words.

"We must be clear-eyed: the world is a more dangerous and uncertain place than at any point since the Second World War. Canada is facing challenges that are unprecedented in our lifetimes."

Thousands gathered along a parade route early Tuesday morning for a chance to see their monarch. The atmosphere was festive with people waving Canadian flags.

Kirsten Hanson, 44, said she welcomes the king's show of support amid the pressure coming from Canada's southern neighbor.

"I think if there's anything that he can do to demonstrate Canada's sovereignty I think that that's fantastic," she told AFP.

"Nobody wants to be absorbed into the US," she said. (AFP)