Detective Chief Superintendent Martin Snowden said both Islamist and far-right groups were increasingly radicalising people through the internet.
He said: “That is where a lot of the radicalisation can occur, from both sides, whether it be of an Islamist nature or an extreme right-wing nature.
“The risk of online radicalisation is probably one of the greatest we face now.
“People can become radicalised in their own bedrooms and there isn’t the need for physical activity or physical meetings.”
Speaking exclusively to The Yorkshire Post, Det Chief Supt Snowden said investigating the dissemination of extremist material online could be extremely time-consuming.
He said if one person sent an extremist video to 100 others, all 100 would have to be assessed by security services to see whether they had forwarded it on.
“It can quickly grow in terms of size and scale,” he said.
Det Chief Supt Snowden said it would be helpful to his team’s work if there was “less of that (material) available to members of the public”.
Concerns over the availability of material such as execution videos, recruitment campaigns and bomb-making instructions on the internet intensified after a wave of terrorist atrocities hit Britain in 2017.
The Home Office said all five attacks on UK soil last year had an “online component”, and major platforms have come under sustained pressure over the issue.
Last month, Home Secretary Amber Rudd unveiled new technology that aims to automatically detect terrorist content before it hits the web.
Tests show the tool can identify 94 per cent of IS propaganda videos and has an extremely high accuracy rate.
A number of Yorkshire people have been brought before the courts in the past year for sharing the online propaganda of so-called Islamic State.
Dr Afshin Shahi, a senior lecturer from the University of Bradford’s peace studies department, compared radicalisation to a contagion which could spread within networks of people, which was why the focus of anti-terror work would change over time and was “sometimes in West Yorkshire, sometimes in South Yorkshire and so on”.
Sheffield found itself the focus of counter-terror operations in December when raids were carried out in the city and nearby Chesterfield, Derbyshire, in response to an alleged Islamist terror plot that could have taken place at Christmas.
Dr Shahi said: “There isn’t anything inherently problematic about any region.
“What makes a region a security issue is the existing networks operating within those regions.
“According to some studies, if you are connected to somebody who is radicalised, you are also more likely to go through radicalisation. That’s why the focus changes.
“For example, if there’s a small network of people in a specific region they often have the ability to grow – that network is radicalised and it is contagious.”
But Dr Shahi was critical of the Government’s anti-extremism Prevent programme, which has faced repeated accusations of demonising British Muslims in recent years.
It was described as a “toxic” brand by the Home Affairs Select Committee in 2016 after MPs heard the testimony of Muslim communities from Bradford, Leeds and Dewsbury, among other places.
Dr Shahi said Prevent, while laudable in its aims of stamping out extremism, had not dealt with its “serious PR problem” and this needed to be tackled urgently if it was to foster an effective relationship with Muslim communities.
He said: “If they cannot maintain a sustainable and trusting relationship with the community, it doesn’t matter how much money they spend and it doesn’t matter what they do, their ability to make meaningful change is going to be undermined.”
The Government has insisted its strategy has had a significant impact in preventing people from being drawn into terrorism.