A passerby on Thursday morning alerted the police after seeing a wartime grenade on the corner of Vaasankatu and Kustaankatu.
Police arrived at the scene after 8am, bringing a specially trained officer in to remove the grenade.
"It's a wartime grenade and the situation is under control," Helsinki Police tweeted on Thursday.
Police are now investigating how the small bomb ended up on the sidewalk.
Helsingin Sanomat reported that a person walking to work on Thursday morning first saw the grenade sitting next to a street power box.