Seven people were arrested, including two from Coventry, on suspicion of funding terrorism, Scotland Yard have said.
Officers from the Metropolitan Police counter-terrorism command arrested six men and a woman at four addresses in London, Coventry and Cardiff.
They are all being held at a central London police station on suspicion of being concerned in the funding of a terrorist organisation and laundering the proceeds of crime for that purpose.
All seven were arrested under the Terrorism Act 2000 as part of a pre-planned, intelligence-led operation, into suspected fundraising for terrorism overseas, Scotland Yard said.
These included a 45-year-old woman and a 49-year-old man in Croydon; two men aged 30 and 47 in Brent; two men aged 42 and 45 in Coventry; and a 40-year-old man in Cardiff.
Homeland Security Investigations in America and officers from the UK's counter terrorism network, the UK Border Agency (UKBA) and local forces were also involved in the operation, Scotland Yard said.
The arrests were part of an investigation into a network suspected of illegally exporting the stimulant khat, which is legal in the UK, to the US and Canada where it is a controlled substance, police said.
A further seven residential addresses in London, Worthing, and Coventry, and a business premises in Coventry are also being searched, but there have been no arrests at these sites.