Anti-terror police in Britain arrested seven people Tuesday over the financing of overseas terrorism as part of a probe with US authorities into a network suspected of exporting the stimulant drug khat.

Early-morning raids at home addresses in London, Cardiff and Coventry led to the arrests of six men and one woman in their 30s and 40s, who are being held on suspicion of financing a terror group and laundering the proceeds of crime.

They are being held in central London while searches under the Terrorism Act are ongoing, London's Metropolitan Police said in a statement.

"The arrests were a part of a pre-planned, intelligence-led operation, into suspected fundraising for terrorism overseas," the statement said.

"The MPS operation, which also involves Homeland Security Investigations in America, is investigating a network suspected of illegally exporting the stimulant khat, which is legal in the UK, to the USA and Canada where it is a controlled substance," it added.

Counter-terror police are searching another seven homes in London, Worthing and Coventry, plus a business address in Coventry, they said.

Five men were arrested in Luton on the outskirts of London last week on suspicion of preparing acts of terrorism, as Britain prepares to mount its biggest peacetime security operation for the London Olympics starting in July.

Khat, a leaf native to east Africa that is traditionally chewed to achieve a mild high in countries including Somalia and Yemen, is imported to Britain mainly for the use of its estimated 100,000 people of Somali origin.