British police arrested 12 men on Monday morning in a large-scale national counter terrorism operation.

The men -- five from Cardiff, four from Stoke and three from London -- were detained on suspicion of commission, preparation or instigation of an act of terrorism in Britain, said a statement released by West Midlands Police.

Aged between 17 and 28, the suspects were detained by unarmed officers at about 0500 GMT.

All were arrested at or near their homes, except the one suspect from Stoke who was at a domestic property in Birmingham. Searches are being conducted in their residences in Birmingham and London.

The suspects will be held at police stations in Central London, the North West and the West Midlands.

The operation was led by senior national coordinator for terrorist investigations and coordinated by West Midlands counter terrorism unit. Counter Terrorism Command of the metropolitan police, South Wales police and Staffordshire police also joined the operation.

Assistant Commissioner John Yates, national lead for counter terrorism policing, said: "This is a large scale, pre-planned and intelligence led operation involving several forces."

"The operation is in its early stages so we are unable to go into detail at this time about the suspected offences. However, I believe it was necessary at this time to take action in order to ensure public safety," he said.