Obiang, son of President Teodoro Obiang Nguema, and two other senior officials were accused of kidnapping four members of the Spain-based opposition group, the Movement for the Liberation of Equatorial Guinea Third Republic (MLGE3R), while they were on a trip to South Sudan in 2019.

In a complaint filed with Spain's High Court in 2020, the MLGE3R alleged that the four - two of whom were Spanish citizens - were sent to Equatorial Guinea, a former Spanish colony, on a government plane and tortured, court document showed.

One of them, Spanish citizen Julio Obama, died last year in prison in Equatorial Guinea. The complaint alleged that the group were also surveilled on Spanish soil.

A spokesman for the Equatorial Guinea government did not immediately respond to a Reuters request for comment. The government has previously rejected the allegations as false.

High Court judge Santiago Pedraz, who was investigating the allegations before deciding whether to proceed to trial, said he was ending the case because Equatorial Guinea's Supreme Court of Justice was conducting its own investigation into the matter, court documents showed.

He said he could not be the judge of "whether those proceedings might suffer from a lack of impartiality".

He also found that there was no evidence that the four victims - all of whom lived in Spain - were previously under surveillance in Spain or coerced into travelling to South Sudan, as the complaint stated, or of any other criminal activity.

The older Obiang has ruled Spain's oil-rich former colony for 44 years and is the world's longest-serving president. He has won all his six terms with more than 90% of the vote in elections whose legitimacy international observers have questioned.

His other son, Teodoro Nguema Obiang Mangue, is vice president.

Rights groups accuse the president of muzzling dissent and cracking down on rivals. Protests are mostly forbidden, media are heavily controlled, and political opponents are often arrested and tortured, they say. The government has denied the accusations.

(Reporting by Emma Pinedo; Editing by Inti Landauro, Aislinn Laing and Alison Williams)