Police on Wednesday arrested 10 people in a sweep targeting support for terrorism outside Britain, Greater Manchester Police said.
About 500 officers were involved in the operation, and searches were continuing at several locations, police said.
Greater Manchester Police said three people were arrested under the Terrorism Act, while five were arrested under immigration laws. Two others were released within hours.
"We are not talking today about a direct threat to the UK," said Michael Todd, chief constable of Greater Manchester Police. "We are talking about the facilitation of terrorism overseas. That could include funding, providing support and encouragement to terrorists."
The Home Office indicated that at least five were not British citizens, and were detained under powers to deport people whose presence "is not conducive to the public good for reasons of national security."