Malaysia has a Muslim majority but most follow Sunni Islam. Shias have long faced discrimination, with religious authorities considering them deviant and imams regularly denouncing them in sermons.
On Monday eight men were detained by religious authorities and police in southern Johor state during a Shia event at a private residence, according to a participant.
"They used violence on us, some of us were handcuffed. They accused us of insulting Islam," cleric Hasan Askari told AFP on Wednesday.
On Friday religious officials and police in central Selangor state arrested 22 Shias at a private property as they recited the Quran and hauled them in to give statements, according to cleric Kamil Zuhairi.
Religious authorities in Selangor confirmed the arrests and said it was because Shia teachings are banned in the state, the official Bernama news agency reported.
All of those detained have now been released but some expressed fears that they could still face action in the country's Islamic courts.
Both events were celebrations related to the Shia holy day of Ashura, which marks the seventh-century killing of Prophet Muhammad's grandson.
"Unless Malaysian authorities, NGOs and civil society respect and tolerate the religious practices of all persons, we cannot truly profess to be a diverse and multi-cultural nation," said Suhakam, Malaysia's official human rights commission.
"The crackdown on Shiites celebrating Ashura must stop. Shiites have the right to commemorate Ashura," said Ahmad Farouk Musa, founder of rights group the Islamic Renaissance Front.
While Sunnis and Shias both follow Islam, they differ in many ways including in law, ritual and doctrine.
About 60 percent of Malaysia's 32 million people are ethnic Malay Muslims and the country is also home to sizeable ethnic Indian and ethnic Chinese communities, who do not usually follow Islam.
