Spain warned it would recall its ambassador from Tehran for consultations if Tehran does not apologise or offer some valid explanation within 48 hours, said Foreign Minister Trinidad Jimenez.
A representative of the foreign ministry said the diplomat, in charge of consular affairs in Tehran, was held as he was in a car heading for the embassy yesterday.
The government summoned the Iranian ambassador "to obtain information and to protest against what we consider a very serious and unacceptable incident," the representative said.
She said Madrid also sent a protest letter to the Iranian foreign ministry over the incident.Jimenez told parliament Iranian police held the diplomat "without taking into account his diplomatic status, thus violating the Vienna Convention" on international law between states.
Spain's leading daily El Pais said on its website that Iranian police accused the diplomat of taking part in a banned anti-government demonstration in Tehran yesterday.
The paper quoted the diplomat, whom it named as Ignacio Perez-Cambra, as saying he was with Spain's ambassador near the demonstration "which we are allowed to do under the Vienna Convention."
Perez-Cambra was held just after leaving the ambassador and was taken by police to police headquarters in Tehran where he was told to hand over his mobile phone.
"They did not mistreat me but there were moments of great tension," he told El Pais by telephone, adding that his mobile phone was protected by a secret code