Iranian defence minister, Amir Hatami said the tanker's seizure "will not be tolerated by us and will not go without a response".
Describing the act as "threatening", he added: "This is an incorrect and wrong action, an action similar to maritime robbery ... certainly these kind of robberies will not be tolerated."
Echoing Hatami's sentiments, Abbas Mousavi, spokesman for Iran's foreign ministry, told reporters the incident was a case of "piracy" and denied allegations that the 330-metre tanker, capable of carrying two million barrels, was taking crude to Syria.
Authorities in Gibraltar, a British overseas territory on Spain's southern tip, say the crude was destined for Syria's Baniyas refinery.
Abbas Araghchi, Iran's deputy foreign minister, told reporters in Tehran on Sunday that "the port named in Syria does not even have the capacity for such a supertanker to dock".
"Its destination was somewhere else," he said, without elaborating.
British Royal Marines, police and customs agents on Thursday stopped and seized the Grace 1 vessel in Gibraltar on suspicion it carried Iranian crude oil to Syria in breach of European Union sanctions against President Bashar al-Assad's government.