In the 1962 Constitution of Burundi, Article 18 provides for freedom of association: "All Barundi have the right of association and of assembly, except in regard to associations or assemblies which are illegal or contrary to morals."

References:

English translation of the Constitution of 1962, "Title II: Barundi and their Rights," Constitution of the Kingdom of Burundi : 20-21: https://heinonline-org.proxygw.wrlc.org/HOL/Page?collection=cow&handle=hein.cow/zzbi0002&id=3&men_tab=srchresults