Authorities confiscated around 100 sheets of sample ballots marked with "YES" to the Bangsamoro Organic Law (BOL) from a man during a routine check in Isabela, Basilan, Tuesday night.
The man, whose identity was withheld by authorities, was intercepted at a checkpoint established by the Philippine National Police (PNP) and the Armed Forces of the Philippines (AFP).
He is now being investigated at the Isabela City Police Station.
On Monday, voters in the Autonomous Region in Muslim Mindanao (ARMM), Isabela City in Basilan, and Cotabato City joined the plebiscite to determine the fate of the BOL.
Another plebiscite is set for February 6 in six towns in Lanao del Norte, 39 barangays in different North Cotabato towns and other contiguous areas that seek to be included in the region.
The Bangsamoro law is the product of a 2014 peace deal between the government and the MILF, which has sought self-governance for the Moro people since the 1970s.