The Imperial War Museum was closed and its grounds evacuated moments before visitors were due to observe a two minutes silence to mark Armistice Day.
Specialist officers attended the scene at 10.52am to reports of an unattended suitcase being found at the Lambeth Road, south London, attraction.
Witnesses have said armed police were at the museum and a bomb disposal robot has been deployed.
A controlled explosion has since taken place on the suspect package and plans for school children to plant poppies at the grounds have been altered.
Visitors have now been allowed back into the museum.