Senators are getting a security briefing from officials of the Department of National Defense (DND), Armed Forces of the Philippines (AFP) and other government officials involved in national security.
This comes in view of the President Rodrigo Duterte's request for Congress to extend martial law in Mindanao for one more year.
Some senators from the minority say, the grant of martial law extension by Senate and the House of Representatives is already a foregone conclusion.
This is because Duterte's allies control the super majority in both chambers.
Nevertheless, opposition senators say, this situation will not prevent them from airing their concerns about the martial law extension.
They also hope some individuals will go the the Supreme Court to question the Commander in Chief's appeal to lawmakers.
On Wednesday, Congress will hold a joint session to make a decision on the matter.