Officers went to Persevere Court in the Leith area of Edinburgh today to search a flat.
Two bomb disposal officers were at the scene, police said.
A number of properties have been evacuated as a "precaution" and surrounding roads have been cordoned off.
A police spokesman said: "As part of an ongoing Police Scotland investigation in Edinburgh, officers within the capital are today conducting a further search of a separate address in Persevere Court.
"During this search, a number of local residents have been asked to leave their homes as a precaution and to allow police and partner agencies to conduct a thorough search of the property.
"We would like to thank those affected by our activity for their patience and co-operation and we are endeavouring to return the area to normality as soon as possible."