West Midlands Police referred inquiries about the operation on Hagley Road to the Metropolitan Police, who refused to say whether it was connected with Wednesday's attack in Westminster.
But a witness who works nearby told the Press Association: "The man from London lived here."
Officers stormed a second-floor flat above a row of shops on Hagley Road at around 11pm last night, the witness said.
He added: "They came and arrested three men."
One of the flat's windows was covered in cardboard, with non-uniformed officers spotted taking pieces of equipment into the property.
As he was describing the events, the witness was interrupted by a police officer, who had been guarding the scene.
He had his details taken and was convinced to go to a police station with another officer.