Filipinos have begun voting in midterm elections highlighted by a showdown between President Rodrigo Duterte's allies who aim to dominate the Senate and an opposition fighting for check and balance under a leader they regard as a looming dictator.

Nearly 62 million Filipinos have registered to choose among 43,500 candidates vying for about 18,000 congressional and local posts in Monday's elections in one of Asia's most rambunctious democracies.

The most crucial race is for 12 seats in the 24-member Senate, which Duterte wants to fill with allies to bolster his legislative agenda that includes the return of the death penalty.

Military and police forces are on full alert to respond to any violence, especially in security hotspots, and help prevent cheating amid intense local political rivalries.