You have to read Section 143 with section 141
Section 141 in The Indian Penal Code
141. Unlawful assembly.—An assembly of five or more persons is designated an “unlawful assembly”, if the common object of the persons composing that assembly is—
(First) — To overawe by criminal force, or show of criminal force, 1[the Central or any State Government or Parliament or the Legislature of any State], or any public servant in the exercise of the lawful power of such public servant; or
(Second) — To resist the execution of any law, or of any legal process; or
(Third) — To commit any mischief or criminal trespass, or other offence; or
(Fourth) — By means of criminal force, or show of criminal force, to any person, to take or obtain possession of any property, or to deprive any person of the enjoyment of a right of way, or of the use of water or other incorporeal right of which he is in possession or enjoyment, or to enforce any right or supposed right; or
(Fifth) — By means of criminal force, or show of criminal force, to compel any person to do what he is not legally bound to do, or to omit to do what he is legally entitled to do. Explanation.—An assembly which was not unlawful when it assembled, may subsequently become an unlawful assembly.
Kindly explain all details fact of the case that how many people where there and for investigation how many police officer where there.