The accusations, which India rejected as "absurd," sent already sour relations between Ottawa and New Delhi to a dramatic new low.

Prime Minister Justin Trudeau told an emergency session of the parliamentary opposition at mid-afternoon that his government had "credible allegations" linking Indian agents to the slaying of an exiled Sikh leader, Hardeep Singh Nijjar, in June in British Columbia.

"The involvement of any foreign government in the murder of a Canadian citizen on Canadian soil is an unacceptable violation of our sovereignty," Trudeau said.

He called "in the strongest possible terms" on the Indian government to cooperate in clearing up the matter.

Foreign Minister Melanie Joly said the Trudeau government had taken immediate action.

"Today we have expelled a senior Indian diplomat from Canada," she said, without naming the official.

Joly said the expelled Indian is the head of the Research and Analysis Wing (RAW), India's foreign intelligence agency, in Canada.

Nijjar, whom India had declared a wanted terrorist, was gunned down on June 18 in Surrey, a suburb of Vancouver that is home to a major Sikh community. 

Canada has the largest population of Sikhs outside of Punjab, India.

Nijjar advocated for the creation of an independent Sikh state to be carved out of parts of northern India and perhaps part of Pakistan. 

New Delhi had accused Nijjar of carrying out terrorist attacks in India, a charge he denied.

India's foreign ministry on Tuesday rejected Canada's claims of involvement in his death.

"Allegations of Government of India's involvement in any act of violence in Canada are absurd and motivated," the ministry said in a statement, adding: "We are a democratic polity with a strong commitment to rule of law."

It said Trudeau -- who visited New Delhi this month for the G20 summit -- had already made similar allegations to Indian Prime Minister Narendra Modi, and that they had been "completely rejected".

Tense Modi, Trudeau G20 meeting 

Tensions between India and Canada have been simmering over the unsolved slaying, and Indian unhappiness over how Ottawa has handled right-wing Sikh separatists.

New Delhi accuses Ottawa of turning a blind eye to the activities of Sikh nationalists who seek a separate Sikh homeland in northern India.

A former adviser to Trudeau, Jocelyn Coulon, asserted that Canada's accusation would have "the effect of a bomb around the world."

India will join "the group of nations that assassinate political opponents" abroad, much as Saudi Arabia orchestrated the murder of journalist Jamal Khashoggi in Turkey in 2018, said Coulon, who is now an independent researcher.

Tensions between the two nations flared during the G20 summit.

Modi expressed "strong concerns about continuing anti-India activities of extremist elements in Canada" during a meeting with Trudeau, according to an Indian government statement. 

India has often complained about activities of the Sikh diaspora abroad, particularly in Canada, which New Delhi believes could revive a Sikh separatist movement.

The Indian state of Punjab, which is 58 percent Sikh and 39 percent Hindu, was rocked by a violent separatist movement in the 1980s and early 1990s, in which thousands died.

Canada also recently suspended negotiations for a free trade agreement with India.

Trudeau later told media that Canada would always defend "freedom of expression, freedom of conscience and freedom of peaceful protest" while acting against hatred.