About 500 members of militant groups have arrived at Mendiola in Manila to hold demonstrations.
They are protesting the alleged dictatorial style of governance of President Rodrigo Duterte.
As of noon on Thursday, militants have occupied one lane of Legarda Street.
In the afternoon, they will walk towards Luneta where they will be joined by other demonstrators.
Malacanang had advised law enforcers to observe maximum restraint in dealing with protesters.
Duterte declared September 21 as National Day of Protest.
It coincides with the 45th anniversary of Former President Ferdinand Marcos' martial law proclamation.