Seven people have been arrested in three cities on suspicion of raising money for terrorism through illegal exports of the substance qat.
The suspects, who include a 45-year-old woman, were arrested as part of an operation run with the US government's department of homeland security and the UK Border Agency, police said.
They are suspected of exporting qat – which is legal in the UK – to the US and Canada, where it is illegal, to fund a terrorist group overseas.
Qat is used predominantly by members of the Somalian community in the UK. The arrests appear to be linked to the alleged funding of al-Shabaab – the al-Qaida-linked terrorist group in Somalia.
The seven are being questioned on suspicion of funding of a terrorist organisation, and laundering the proceeds of crime for that purpose, contrary to the Terrorism Act 2000.
The arrests come as police and security services are on high alert in the runup to the Olympics and as a major military exercise takes place across the capital in preparation for the Games.
The suspects were arrested at four residential addresses in London, Cardiff and Coventry at approximately 6am on Tuesday, Scotland Yard said.
They are a 45-year-old woman and a 49-year-old man, who were both arrested in Croydon, south London, two men aged 47 and 30 who were arrested in Brent, north London, two men aged 45 and 42 arrested in Coventry and a 40-year-old man arrested in Cardiff.
A spokesman for the Metropolitan police said: "The co-ordinated operation is led by the Met's counter-terrorism command, with assistance from the counter-terrorism network, the UK Border Agency and local forces.
"The arrests were a part of a pre-planned, intelligence-led operation, into suspected fundraising for terrorism overseas.
"The operation, which also involves homeland security investigations in America, is investigating a network suspected of illegally exporting the stimulant qat, which is legal in the UK, to the USA and Canada where it is a controlled substance."
Searches of the homes were under way, the police said, and searches of eight further properties – residential addresses in London, Worthing and Coventry and a business premises in Coventry – were also taking place, the police said.
Several Britons are known to have travelled to Somalia for training with al-Shabaab. In a separate ongoing investigation, British police are liaising with the authorities in Kenya in the hunt for two Britons who are allegedly involved in the organisation, including the widow of the 7 July suicide bomber Germaine Lindsay.