LONDON - British police have foiled four or five suspected terror plots this year, the country's most senior police officer said Sunday ahead of a week-long campaign to enlist the public's help in countering the threat.

"We've said on average over the last few years it's been about one (plot) a year, but this year alone we think four or five," Scotland Yard commissioner Bernard Hogan-Howe told BBC television.

He added: "Certainly we've seen a change to the momentum... we've seen a change to the frequency and the seriousness of the types of plots that we're looking at." More than 250 counter-terrorism arrests have been made so far this year, according to police sources. Most recently, three people were charged in London last week over an alleged plot which media reports said included a plan to behead somebody in Britain.

Hogan-Howe said the threat from so-called "lone wolf" attacks by individuals or small groups was causing "growing concern", and warned their ability to act quickly left little time for the security services to intervene.

But he said the public could help by being more vigilant, particularly in crowded places and transport hubs where attacks are more likely to happen.

For a week starting on Monday, police will hold a series of events across Britain to let ordinary people and businesses know how they can help by identifying and reporting suspicious behaviour.

The campaign will also urge people to question charities about where their money is going, amid concerns that some are being used to channel funds to militants.

"If the public, the businesses and police work together with the security services then that's an incredibly powerful team," Hogan-Howe said.

The national terror threat was raised in August to "severe", meaning an attack is highly likely, but the police chief said the message was to "keep calm but be aware".

He repeated fears that Britons who have gone to fight with militant groups in Iraq and Syria might return to use their new training and experience to attack Britain.

But he did not comment on a claim by an opposition Labour lawmaker that as many as 2,000 Britons are fighting overseas - four times the official estimate of 500.

Khalid Mahmood, an MP for the city of Birmingham, told the Sunday Telegraph newspaper that in his area - which has a significant number of Muslims - there was a "huge problem" of people going to fight with the Islamic State (IS) group.