A young man has been arrested in east London on suspicion of the commission, preparation or instigation of terrorism – as four people are also held in Yorkshire.
The 23-year-old was detained by officers from the Metropolitan Police's Counter Terrorism Command on Wednesday morning.
Meanwhile, in an unconnected operation four people – two men aged 36 and 40, from Sheffield, and a 55-year old woman and 44-year old man from Batley, West Yorkshire – were arrested on suspicion of Syria-related terror offences.
The North East Counter Terrorism Unit said they were also held by officers on Wednesday morning.
In east London, officers attended the residential address and arrested the man section 41 of the Terrorism Act 2000.
They are continuing to search the address and a second residential address in east London.
A Scotland Yard spokesman said: “The man is being detained under Schedule 8 of Terrorism Act 2000 legislation. Enquiries continue.”
The two Sheffield men were arrested on suspicion of an offence under Section 5 of the Terrorism Act – alleged preparation to commit an act of terrorism.
The man and woman from Batley were arrested on suspicion of an offence under Section 15 of the Terrorism Act – alleged terrorist fundraising.
They have been taken to a police Station in West Yorkshire for questioning.
As part of the investigation, five properties in Sheffield, Batley and Dewsbury are being searched, a spokesman said.
They added: “The investigation is Syria related.”
Det Ch Supt Clive Wain, head of the North East Counter Terrorism Unit, said: “We understand that people may be concerned following today's arrests however we would like to reassure communities that today's activity is as a result of an ongoing investigation which is intelligence-led.
“There is no evidence to suggest that communities are at risk.”
Earlier this year, Home Secretary Theresa May said Britain is facing its “greatest terror threat in its history”.
In an interview with The Telegraph, she said the menace posed by the Islamist faction means the threat to Britain is now “greater than it has been at any time before or after 9/11”.