LONDON (AFP) - British police on Monday urged the public to be alert to potential terrorist activity as they said the threat of extremist attacks would last for several years.
Launching Counter-Terrorism Awareness Week, police said that even if the violence in Iraq and Syria subsided, the risk of attacks in Britain would continue.
The week of events, which will include talks with university students, businesses, theatre group performances in schools and social media interaction.
It comes amid heightened concerns over the numbers of British extremists joining forces with counterparts in Iraq and Syria - and potentially returning home with terror "tradecraft".
"The danger posed by violent extremists has evolved," said Mark Rowley, Britain's chief counter-terror police officer.
"They are no longer a problem solely stemming from countries like Iraq and Afghanistan, far away in the minds of the public.
"Now, they are homegrown, in our communities."
He said several attack plots had been disrupted in 2014 and more than 270 arrests had been made this year following counter-terror investigations.
Interior minister Theresa May said on Monday that since the government came to power in 2010, a total of 138 people have been put behind bars for terror-related offences.
She also said she had revoked British citizenship for 27 dual nationality holders in the same period, as well as refusing or cancelling 29 passports to prevent people travelling to fight in Syria and Iraq.
However, "the eyes and ears of law enforcement and other agencies alone cannot combat the threat", Rowley warned.
"We don't want to scare people but we do want them to understand the threat and be vigilant to things that are out of place or suspicious and report it to the police."
He said militants target busy, well-populated places for maximum impact, so urged businesses to check their security measures and train staff to spot potential threats, as they are often the first to notice when things are amiss.
The official terror threat level in Britain was raised to "severe" in August, the second-highest of five levels, meaning that an attack is considered "highly likely".
Across Britain, officers will be briefing more than 6,000 people at 80 venues during Counter-Terrorism Awareness Week.
Events will be held in schools, airports, shopping centres, cinemas and farms in a bid to engage people in helping to prevent attacks.