"I want to show my most absolute rejection of the actions of the Public Ministry of Guatemala that threaten democracy and the will of the citizens expressed at the polls. Democracy must prevail," Pedro Sanchez posted on social media site X, Reuters reported.
Arevalo on Friday rejected a legal manoeuvre from prosecutors to invalidate his election triumph, calling the effort "perverse" and an "attempted coup".
Ever since anti-corruption crusader Arevalo did much better than expected in June's first-round election, qualifying for the decisive August runoff, he and his centre-left Seed Movement party have faced a series of investigations from the attorney general's office, which has alleged irregularities in the party's registration several years ago.
Spain, which holds the Council of the European Union's rotating presidency, has sought to improve relations between the bloc and Latin America.
Spain still hoped to clinch a trade deal between the EU and South American trade bloc Mercosur bloc by year-end, a Spanish Economy Ministry source said on Dec. 4.
The EU and the Mercosur bloc of Argentina, Brazil, Paraguay and Uruguay have been negotiating a deal for almost 25 years.
Spanish investors play an important role in Latin America, particularly Telefonica which has had commercial ties to the continent for the past 30 years.