British police arrested 10 men in counter-terrorism raids across the northwest of England today, police said.
Police in the northern cities of Liverpool and Manchester were taking part in the operation and said further details would be available later.
"A number of addresses across the northwest are currently being searched," police said in a statement.
Television news footage showed police making arrests at John Moores university in Liverpool and an internet cafe in Manchester.
British media reported the arrests may have been brought forward because of fears of a security leak involving Britain's most senior counter-terrorism police officer.
Police tend to carry out anti-terrorism raids in the early morning rather than late in the afternoon.
Earlier yesterday, Scotland Yard's Assistant Commissioner Bob Quick was photographed carrying sensitive papers outside Prime Minister Gordon Brown's offices in London.
"Assistant Commissioner Quick accepts he made a mistake in leaving sensitive documents on open view," Scotland Yard said.
"He deeply regrets it and has apologised to the Commissioner and colleagues."