Police are “relying on the public” to spot suspected terror activity during the festive period as tip-offs rise by half, a senior officer has said.

Counter-terrorism officers are now “most concerned” by lone actors as opposed to well-organised groups, Metropolitan Police deputy assistant commissioner Jon Savell said.

As a result, the public acting as officers’ “eyes and ears” is now “the fundamental thing” in policing terrorism, he added, speaking in his role as senior national co-ordinator for the Government’s “protect and prepare” counter-terrorism strategy.

This year has seen a 50% rise in suspicious activity reports made to the anti-terror hotline. The cause of the spike is uncertain but contacts are making a difference to investigations, according to Counter Terrorism Policing (CTP).

There were 52 more arrests for terrorism offences last year than in 2022 – an increase of 31% and the highest number in a single year since 2019.

Christmas shopping, festive markets, pantomimes and concerts could attract terror activity, CTP warned, as it reminded people “everyone has a role to play in keeping each other safe” by spotting and reporting suspicious activity.

Mr Savell walked through a busy Christmas market at London Bridge with security minister Dan Jarvis on Thursday afternoon, discussing terror policing during the winter months.

After the meeting, Mr Savell told the PA news agency: “Things have changed from the historically sort of well-organised group, hostile state-supported terrorism, to lone actors.

“We get a lot of people now that are getting information off the internet, they’re sitting at home finding out this information, becoming radicalised online.

“They’re the ones I think we’re the most concerned about – what we call low-sophistication, so using vehicles, knives, not firearms or bombs that we traditionally might have seen.”

Asked how the shift is influencing policing tactics, he said: “I think the fundamental thing is actually relying on members of the public, so that’s what we need to do – the public are our eyes and ears. Without public support and vigilance, it actually makes our job an awful lot more difficult.

“Don’t worry (if you) think (you’re) going to be getting somebody in trouble, let us be dealing with that.

“People spotting individuals that are doing what might be hostile reconnaissance, or are looking like they’re up to no good? Tell us.”

Last year saw the highest number of young people arrested for terrorist offences since records began on September 11 2001, according to CTP.

A total of 19% of the 219 terror-related arrests made last year were young people aged 17 and under.

If you have any concerns about the activity you see, whatever it might be, then please do flag that with the police, and then they can take a decision about how best to proceed and respond to it

Mr Savell said: “We’re certainly seeing a lot more young people becoming involved, being radicalised, seeing things on the internet that actually are changing their ideology and their thoughts, and they’re young and they’re vulnerable very often.

“I think social media, as valuable as it is, can also be incredibly destructive, and so easily prevalent and very difficult to control, and I think that’s our biggest issue.”

Counter-terror police are seeing a rapid increase in the number of suspects fascinated with extreme violence, a senior officer Met officer said on Wednesday.

Asked if there is anything driving the trend beyond social media, Mr Savell said: “I don’t think there’s anything else that I would particularly be highlighting.

“It’s for us and others to be vigilant about what their friends, their children, their partners, are doing and seeing on social media, and giving some advice to them.”

CTP said it has more than 800 investigations under way, most in partnership with MI5, and it is working with intelligence services at MI5 and GCHQ to understand and prevent radicalisation.

It has stopped 43 late-stage attacks from taking place alongside MI5 – three were in the last 12 months.

Security minister Mr Jarvis told PA: “Police do a fantastic job to keep people safe, but we’re asking the public just to remain vigilant – to keep an eye open if they see anything that they think is suspicious, to report that to the police.

“The public are very sensible, they will take a judgment based on their intuition as to whether something that they see is unusual or not.

“If you have any concerns about the activity you see, whatever it might be, then please do flag that with the police, and then they can take a decision about how best to proceed and respond to it.”

The current terror threat is “substantial”, meaning an attack is likely.