Official results from the Philippines Election Commission shows key allies of detained former President Rodrigo Duterte and two liberal opposition candidates are among top winners in the nation's midterm Senate race.

Former Philippine President Rodrigo Duterte is set to return to his longtime political stronghold as Davao City mayor, winning by a landslide despite being detained at the International Criminal Court (ICC) in The Hague.

Despite the ICC's case also including alleged killings of criminal suspects by a "death squad" in Davao while Duterte was mayor - which he has denied - analysts have said his arre