Police were called to a building site in Empire Way shortly before 3pm on Thursday (May 21).
Officers have implemented a 400m cordon around the site while they work to make the device safe, and are liaising with partners to assist those who have been displaced. It was still in place on Friday (May 22) morning.
Transport for London (TfL) is warning drivers to expect delays as the road has been closed in both directions between Wembley Hill Road and Wembley Park Drive.
Brent Civic Centre, in Engineers Way, was among the buildings evacuated.
Brent Council tweeted: "Due to an unexploded WW2 bomb Brent Civic Centre and surrounding buildings have been evacuated. Please avoid the area."