Ana Pastor and Madrid mayor Manuela Carmena were subject to threats and insults in messages revealed from a private police Whatsapp group by newspaper El Diario.
The arrest meant the neutralization of a "potential threat", the Policia Nacional said.
The former mayor of Caracas, a staunch opponent of Venezuela's President Nicolas Maduro who fled to Spain over the weekend, vowed on Monday to battle against the president's regime, warning him to "get ready".
About 1.38 million hectares (3.4 million acres) of grains, sunflowers and olive trees have been affected by drought or frost in Spain as of the end of October, according to Spanish farming insurance agency Agroseguro.
Jurgen Klopp insisted Philippe Coutinho's mind is only on Liverpool ahead of the Brazilian's first appearance on Spanish soil since a prolonged transfer pursuit by Barcelona failed to prise the playmaker away from Anfield.
Spain's central government and Catalan separatists traded the blame Monday after Barcelona failed to be picked to host the European Medicines Agency, which is leaving London after Britain's decision to exit the EU.