Answer
The opposition to Yazid I's succession primarily came from Husayn, the grandson of the Prophet Muhammad, and Abd Allah ibn al-Zubayr, a prominent figure from Mecca. They objected to Yazid's nomination as heir apparent by the Umayyad caliph, Muawiya, citing concerns over his character, lack of qualifications, and suitability for leadership. This dissent laid the groundwork for the subsequent conflicts that would shape the early Islamic caliphate.