Iran Press/ Europe: The blast occurred when an embassy employee was "handling a letter," the Spanish Interior Ministry said in a statement to the media.
The man was only slightly injured but went to hospital unassisted.
Spanish police have activated the anti-terrorist protocol and have cordoned off the area where the Ukrainian embassy is located in the northeast of the Spanish capital.
Forensic police are also said to be participating in the investigation.
Ukrainian Foreign Minister Dmytro Kuleba has ordered the strengthening of the security of all embassies, a spokesperson wrote on Twitter.
"Whoever is behind this explosion will not be able to intimidate Ukrainian diplomats or stop their daily work to strengthen Ukraine and counter Russian aggression," a statement added.
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