Three properties in the Bradford area are being searched by West Yorkshire police following the arrest of a man under the Terrorism Act.
Police guard one of the houses in Hanover Square after the raids
The 21-year-old man, from Bradford, was detained at Manchester Airport on Tuesday night. He is believed to have recently been in Pakistan.
He is being questioned in connection with arrests in Canada over an alleged al-Qaeda plot, sources told the BBC.
Police have confirmed armed officers were not involved in his arrest.
The operation involved West Yorkshire Police and MI5, as well as the Metropolitan Police, but is not being linked to the investigation in Forest Gate, east London.
The man, who is believed to be of Pakistani origin, is now being questioned at a West Yorkshire police station.
Two of the properties being searched are in Hanover Square in the Manningham area of Bradford, while the third is in Otley Road.
Hanover Square neighbour Asad Khan said the noise of banging woke him up just after midnight.
"I came outside to have a look and there were just police everywhere. They were all over. The road was blocked.
"There were two houses that were being raided. The whole neighbourhood had woken up. They were all outside.
"We saw ladies were taken out and put into cars. We don't know how many or who was taken out," Mr Khan said.
"We've never had any trouble like this before. It was quite shocking."
A next-door neighbour of one of the houses raided in Hanover Square said he had seen its owner, an elderly woman, being led away by police along with her son and his wife.
Police had used a battering ram to force the house's door down, he added.
Police confirmed a number of people were removed from addresses so that searches could be carried out.