Patrols by armed police will be increased and security plans for major events reviewed after the terror threat level was raised following the Liverpool attack.

Assistant Commissioner Matt Jukes, the head of UK counter-terrorism policing, said the public will see an increased police presence in key locations and officers will boost their work online.

Speaking in Leicester Square in London’s West End on Wednesday, Mr Jukes told the PA news agency: “All across the country you will have seen plans for major events reviewed, you will see an increased armed policing presence in some key locations.

“In places like London where we have dedicated counter-terror patrols they will be very focused.

“You will see some more visible policing, but of course also we will be very active in other places. Online, in our communities talking to people about their concerns.”

The terror threat was raised from substantial to severe, meaning an attack is “highly likely” rather than “likely”, after the blast outside Liverpool Women’s Hospital on Remembrance Sunday.

Emad Al Swealmeen spent months planning his bomb attack, and had been buying components “at least since April”, investigators said.

Mr Jukes said attacks that take months of planning provide opportunities for the public to spot suspicious behaviour.

“In this case it’s too early to say what the behaviour was and how that might have been spotted by others, or indeed what others saw,” he said.

“But what we do know from other cases is that often attacks are planned over a series of months and there are opportunities for people to see changes in behaviour, to see unusual things happening.

“Unusual deliveries to an address or unusual purchases. Things which seem out of place.

“Without commenting on the individual case, there are so many opportunities for members of the public – neighbours, friends and family – to play their part in keeping our communities safe.

“There are signals in the way people bring together improvised explosives that are important for the public to recognise. Making strange purchases.

“If you’re a retailer, for example, if someone’s buying something that just doesn’t feel right for them, then it’s really important that you make that call to us on the anti-terrorist hotline.”

The senior officer, who has worked in counter-terrorism for two decades, said that even with lone wolf terrorists there are still chances for someone to unmask them.

“The lone actor is still a real concern for us. The individual who spends time online. But the reality is they’re never really completely alone – inspired by something they see online, instructed by something they’ve seen online, and also often in contact with friends and family.

“So it’s so important that friends, family, neighbours spot the changes in behaviour. Very often when we look back on these events there may have been a signal that’s been an opportunity.”

He said police and the security services also need the help of the public, particularly parents, in spotting when someone has accessed terrorist or extremist material online.

“We’re very concerned about the availability of information that’s useful for terrorists online and that’s why every year we’re prosecuting people for sharing that information,” he said.

“We really do again need the help of the public who come across that. Sadly we need the help of parents as well. We know that many, many more young people are finding or coming across some of that information so it’s an important conversation to have.”

UK counter-terrorism police, who along with the security services are working on around 800 live investigations, want the public to remain vigilant about the risk of terrorism in crowded places in the run-up to Christmas.

“This is certainly a very concerning period that has led to the increase in the threat level. What we do know is that sometimes a terrorist attack in the UK or around the world can galvanise or encourage somebody else.

“Oftentimes we do see attacks coming together. That’s the reason why the threat level has been increased.

“Clearly it’s concerning but it’s important for people not to feel powerless in the middle of that and to recognise we’ve all got incredible potential to make a contribution to make that call.

“If you feel something isn’t right, make that call to the anti-terrorist hotline, go online to one of the reporting mechanisms, because we can all make a difference and defeat terrorism.”

The Anti-Terrorist Hotline is on 0800 789 321.