The Bill of Rights Art. III Sec. 5 of the 1987 Constitution declares--
(5a) No law shall be made respecting an establishment of religion, or prohibiting the free exercise thereof. (Principle of State Neutrality)
(5b) The free exercise and enjoyment of religious profession and worship, without discrimination or preference, shall forever be allowed. (Principle of Equal Liberty)
(5c) No religious test shall be required for the exercise of civil or political rights. (Principle of Secular Morality)It would not constitute legitimate exercise of free speech, in my opinion, if persons not allied with an organization that was conducting such a mass demonstration, were to demand a place on the stage or in the proceedings just by showing up, and then unfurl some symbol or message that could cause tumult or disorder. For example anti-communist activists like ANAD have no legitimate place in a Bayan Muna rally if they want to display slogans against JoMa or the NPA. But they may do so at their own rally and can expect a similar exclusion of any hypothetical pro-communist activists bent on provoking a confrontation.
Clearly, organizers have a right to police their ranks and their venues to prevent the unruly presence of PROVOCATEURS bent on disturbing the peace. It's like crying "Fire!" in a crowded theatre, especially if provocation leads to a violent confrontation. Such persons however, have an equal right to make their own mass demonstration, and there engage in their own free speech.