Eight people are being held following an anti-terrorist police operation across England targeting people suspected of plotting attacks abroad.
Addresses have been searched under the Terrorism Act
The raids on 19 addresses, which involved 500 officers, are believed to relate to activities in Iraq.
Police in Greater Manchester, the West Midlands, Cleveland, London and Merseyside began the raids at 0300 BST.
Three people were detained under anti-terrorism legislation and five under immigration laws.
Two other people were arrested, but later released.
The Home Office confirmed five of the people held were foreign nationals detained under the home secretary's powers to "deport individuals whose presence in the UK is not conducive to the public good for reasons of national security".
BBC News understands that one of the men arrested in Manchester is Tahir Nasuf, a 44-year-old born in Libya.
He moved to Manchester in 1993 and is married with four children.
The offices of a charity he works for - the Sanabel Relief Agency - were also raided in Birmingham.
The operation is being led by Greater Manchester Police's (GMP) anti-terrorist unit, and the searches using warrants issued under the Terrorism Act 2000, are likely to continue for at least the rest of the day.
The BBC's Nick Ravenscroft said it was beginning to emerge that the raids probably centred on activities in Iraq.
GMP chief constable Michael Todd said there was no threat of an attack on the UK.
The operation was jointly organised with MI5 and followed an intelligence-gathering investigation that has been going on for "at least a year", said Mr Todd.
MI5 recently established a regional headquarters in the north-west of England.
Police said 18 addresses had been raided - 12 in Greater Manchester, one in Liverpool, one in Middlesbrough, three in Birmingham and one in Eastham, London.
Police said they issued a search warrant for a further property in Farnworth, Bolton, on Wednesday afternoon.
A car parked outside one property raided in Moss Side, south of Manchester city centre, was towed away during the night.
Three people were held under the Terrorism Act and three under immigration legislation in Greater Manchester, one in Merseyside under immigration legislation and one in London under immigration legislation, police said.
POLICE FORCES INVOLVED
Police would not disclose where the ninth arrest was made.
One house in Middlesbrough had been raided and is still being searched, but no arrests have been made, said Cleveland Police.
The Met's anti-terrorist squad confirmed it was involved in the West Midlands operation, in which a property was searched but no arrests made.
Raids in Greater Manchester included property searches in the Whalley Range and Fallowfield areas.
A spokeswoman for the Greater Manchester force confirmed an "extensive operation targeting individuals suspected of facilitating terrorism abroad" was under way.
She would not confirm the exact nature of the alleged offences.