Answer
The most prominent of those who opposed Mu'awiya after his death was Ali's son Husayn. Husayn and his supporters were killed at the Battle of Karbala in 680, which is considered one of the most important events in Shia Islam. Abd Allah ibn al-Zubayr, a cousin of Muhammad, also opposed Mu'awiya. He declared himself caliph in 683 and ruled in Mecca until his death in 692.