Canadian police accused Indian diplomats and consular staff of “clandestine” and “criminal” activities in the country on Monday night, hours after senior diplomats were expelled from both countries in an escalating geopolitical row.

Speaking to reporters at a hastily organised press conferencet, the head of the Royal Canadian Mounted police (RCMP) said the force had evidence of “agents” acting on behalf of the Indian government engaging in extortion, intimidation, coercion and harassment.

Commissioner Mike Duheme told reporters: “Investigations have revealed that Indian diplomats and consular officials based in Canada leveraged their official positions to engage in clandestine activities, such as collecting information for the government of India, either directly or through their proxies; and other individuals who acted voluntarily or through coercion.”

The revelations from the RCMP are the first official glimpse from police into the scope and depth of India’s alleged activities in Canada, which have rocked the country’s politics ever since the prime minister, Justin Trudeau, accused India of assassinating the prominent Sikh leader Hardeep Singh Nijjar in the province of British Columbia last year.

Trudeau said on Monday night: “We will never tolerate the involvement of a foreign government threatening and killing Canadian citizens on Canadian soil. India has made a monumental mistake in choosing to use their diplomats and organised crime to attack Canadians.”

Canada’s foreign minister, Mélanie Joly, said the RCMP had gathered “ample, clear and concrete evidence which identified six individuals as persons of interest in the Nijjar case”.

She said India was asked to waive diplomatic and consular immunity and cooperate in the investigation but refused to cooperate.

“Regrettably, as India did not agree, and given the ongoing public safety concerns for Canadians, Canada served notices of expulsion to these individuals,” Joly said.

She asked that India’s government support the ongoing investigation “as it remains in both our countries’ interest to get to the bottom of this”.

In recent months, activists have accused India of a carefully orchestrated campaign of “transnational terrorism” that targets Sikhs.

Duheme did not indicate how many killings Indian officials had been linked to in Canada, but said the “breadth and depth of criminal activity” was a threat to the safety of Canadians.

Senior Canadian national security and intelligence officials presented their Indian counterparts with their findings over the weekend, including evidence that allegedly implicated India’s high commissioner, Sanjay Kumar Verma, in the killing of Nijjar, a source familiar with the investigation said.

Assistant Commissioner Brigitte Gauvin told reporters the actions of Indian diplomats were “a contravention of the Vienna convention on diplomatic relations, but also it goes against Canada’s values as a society”.

In a statement earlier on Monday, India’s foreign ministry said it had no faith in the Canadian government’s ability to assure the security of its top diplomats and had “decided to withdraw the high commissioner and other targeted diplomats and officials”.

It also announced that six top Canadian diplomats would be expelled from New Delhi in response.

However, Canada’s Globe and Mail newspaper and the Associated Press quoted unnamed Canadian officials alleging that it was Canada that had expelled the Indian diplomats first, after evidence implicated Verma and six top diplomats in the assassination of Nijjar.

Nijjar, a Canadian Sikh, was gunned down outside a gurdwara in the city of Surrey in June last year.

He had been a vocal advocate for the Khalistan movement, which advocates for an independent homeland for Sikhs and is banned in India. The Indian government had accused Nijjar of involvement in Khalistani terrorism.

In the aftermath of Nijjar’s killing, Canada’s prime minister, Justin Trudeau, took the unusual step of publicly announcing there were “credible allegations” that the Indian government was involved in Nijjar’s assassination. India denied the charges, calling them “absurd”.

On Monday, India’s foreign ministry said it “strongly rejects these preposterous imputations” after a diplomatic communication from Canada confirmed that Indian diplomats were cited as “persons of interest” in the investigation into Nijjar’s killing.

The Indian government said the allegations were “ludicrous and deserve to be treated with contempt” and summoned Canada’s charge d’affaires in New Delhi, Stewart Wheeler, to express that the targeting of its diplomats was unacceptable.

The Indian ministry spokesperson accused Trudeau’s government of a political agenda and claimed Canada had not shared any further evidence about Indian state involvement in the fatal shooting since Trudeau had made the allegations in September 2023.

“This leaves little doubt that on the pretext of an investigation, there is a deliberate strategy of smearing India for political gains,” the Indian ministry said.

However, according to reports in Canadian media, the Canadian government presented evidence to India last week but was met with denials by the government of Narendra Modi.

Nijjar’s murder resulted in a significant chilling of diplomatic relations between India and Canada. It also prompted officials and activists to accuse the Modi government of carrying out a campaign of transnational killings against those it considered to be threats to the state.

The accusations were further fuelled last November after US agencies said they had thwarted an assassination attempt by an Indian government official to murder Gurpatwant Singh Pannun, a firebrand Sikh separatist and dual citizen of the US and Canada.

Other prominent Sikh Khalistani activists in the US, Canada and the UK also said they had also been given warnings of threats to their lives.

Last year, the White House said it took the allegations of India’s involvement in attempted killings on US soil “with utmost seriousness” and confirmed it had been raised at the highest levels of the Indian government.

The Indian government set up its own investigation which, according to unnamed officials, recently concluded that the attempted assassination of Pannun was the work of “rogue agents”.

According to intelligence officials who spoke to the Guardian this year, India had also ordered the assassination of dozens of individuals in Pakistan as part of a wider strategy to eliminate terrorists living on foreign soil.

In May, three Indian nationals who had been living in Canada on student visas were charged as the alleged shooters who carried out Nijjar’s assassination. However, police emphasised that the investigation was continuing and “these efforts include investigating connections to the government of India”.