“I saw that the pin was out and the hand grip was up,” Peter Ekman told local reporters.
Several windows were shattered by the force of the explosion, reports Swedish Radio Västerbotten.
The police have not given details of any suspect.
The Left Party leader Jonas Sjöstedt gave his summer speech in Umeå's old town on Sunday afternoon. It is unclear whether the hand grenade could be linked to a threat to the party leader. A press spokesperson refused to comment.