Seven people have been arrested on suspicion of funding terrorism, Scotland Yard 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 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.