SYDNEY - A police operation was underway at the Sydney Opera House on Thursday, police said, with at least one ferry leaving the area cancelled and media reporting that people were being cleared away from the site.A police spokeswoman said an operation was going on but could not provide any details. The Twitter account of the city's ferry service said one ferry service had been cancelled due to the operation.Australia is on heightened alert for attacks by home-grown militants. Authorities say they have thwarted a number of potential attacks, although there have been several "lone wolf" assaults.