India and Canada have expelled their top envoys along with other diplomats as the row intensifies over last year's assassination of a Sikh separatist on Canadian soil.

Canadian Prime Minister Justin Trudeau said his government responded after police began pursuing credible allegations that Indian agents were directly involved in the killing of Hardeep Singh Nijjar.

Canadian police accused Indian agents of involvement in "homicides, extortion and violent acts" and targeting supporters of the pro-Khalistan movement, which seeks a separate homeland for Sikhs in India.

India has rejected the allegations as "preposterous", accusing Trudeau of pandering to Canada’s large Sikh community for political gain.

Speaking on live television on Monday afternoon, Trudeau said India had made a "fundamental error" in supporting "criminal" acts in Canada and his government had to act on the latest findings.

"The evidence brought to light by the RCMP [Royal Canadian Mounted Police, Canada's national police service] cannot be ignored," the prime minister said.

"It leads to one conclusion, it is necessary to disrupt the criminal activities that continue to pose a threat to public safety in Canada. That is why we acted." 

India has vehemently denied all allegations and maintained that Canada has provided no evidence to support its claims.

Relations between Delhi and Ottawa have been strained since Trudeau said Canada had credible evidence linking Indian agents to Nijjar's murder.

The row led to a deterioration in ties, with India asking Canada to withdraw dozens of its diplomatic staff and suspending visa services.

On Monday, a furious statement from India's foreign ministry said Canada's allegations were influenced by Sikh separatist campaigners.

Later in the day, it announced six Canadian diplomats, including acting High Commissioner Stewart Ross Wheeler, had been asked to leave India by 19 October.

Mr Wheeler was also summoned by India's external affairs ministry to explain Canada's move.

Speaking to reporters after the meeting, Mr Wheeler said Canada had given India the evidence it had demanded, it now needed to investigate the allegations.

"It is in the interests of both our countries and the peoples of our countries to get to the bottom of this," he said.

Delhi has defended its High Commissioner Sanjay Kumar Verma, referring to his "distinguished career spanning 36 years".

"The aspersions cast on him by the government of Canada are ludicrous and deserve to be treated with contempt," it said.

The Indian foreign ministry also said it was "withdrawing" its top envoy and other diplomats.

"We have no faith in the current Canadian government's commitment to ensure their security. Therefore, the government of India has decided to withdraw the High Commissioner and other targeted diplomats and officials."

Earlier on Monday, Canadian police said they had taken the unusual step of publicly disclosing information about ongoing investigation "due to significant threat to public safety in our country".

RCMP commissioner Mike Duheme told reporters at a news conference on Monday that there had been "over a dozen credible and imminent threats to life" which he said "specifically" focussed on members of the pro-Khalistan movement.

He added that the threats were sufficiently serious to warrant the RCMP's public intervention.

"We reached a point where we felt it was imperative to confront the government of India."

Officials said a dozen Indian agents were involved in the alleged criminal activities, but did not confirm if they were directly linked to the June 2023 murder of Sikh separatist leader Nijjar .

Hardeep Singh Nijjar was shot and killed by two masked gunmen outside a Sikh temple he led in Surrey, British Columbia.

He had been a vocal supporter of the Khalistan movement, which demands a separate Sikh homeland, and publicly campaigned for it.

India has in the past described him as a terrorist who led a militant separatist group - accusations his supporters called unfounded.

Canadian police called his killing a "targeted attack".

In September 2023, Trudeau had told Canada's parliament that allegations of Indian involvement in the killing were based on Canadian intelligence.

He called the act a violation of Canada's sovereignty.

Frosty ties between the two countries seemed to have thawed slightly after India resumed processing visas in October 2023.

But last week, Canadian Foreign Minister Mélanie Joly called the country's relations with India "tense" and "very difficult".

She also said there remained a threat of more killings like Nijjar's on Canadian soil.

Canada is home to the largest Sikh community outside India, a religious minority that lives mostly in the state of Punjab.