KUALA LUMPUR, Aug 11 (Reuters) - Islamic authorities in Malaysia have freed a Muslim woman after detaining her for four months for marrying a Hindu, the couple's lawyer said on Saturday.

The Selangor state Islamic authorities have, however, ordered the 25-year-old ethnic Indian woman to live separately from her husband, arguing that her year-old marriage was illegal under Islam.

In Malaysia, Muslims cannot marry non-Muslims.

Her case is the latest strain in the social fabric of the multiracial nation, where many non-Muslims believe the authorities and the courts are allowing their rights to be trampled by the Muslim majority.

"Her husband's family rang us up to say she has been released about three weeks ago," said Karpal Singh who had argued in the past that Najeera Farvinli Mohamed Jalali's detention was illegal because no detention order was served on her.

Her husband, a 25-year-old truck driver Magendran Sababathy, had hired Karpal to fight for his wife's freedom.

Islamic religious police had raided the couple's house in April and arrested her, arguing she was "illegally cohabiting" with a Hindu and for for failing to produce any relevant marriage documents.

A prominent rights lawyer questioned the decision to separate Najeera from her husband.

"I don't think there's a legal basis for them to do it," Malik Imtiaz Sarwar said. "I don't know whether they can do that. I don't see where the powers to do so is"

"If she is saying she is not a Muslim, the Constitution guarantees her right to say that and nobody can order her to do anything other than that."

Officials could not be immediately reached for comment on the woman's release and the order that she stay with her parents.

Just over half of Malaysia's 26 million people are Malays, who are Muslims by definition. Ethnic Chinese and Indians form the sizeable minorities and they practice either Buddhism, Christianity or Hinduism.

Last month, a 29-year-old woman said she was mentally tortured by islamic religious police during her six-month detention for renouncing Islam in favour of the Hindu religion.

In May, the country's best-known Christian convert, Lina Joy, lost a battle in Malaysia's highest court to have the word "Islam" removed from her identity card. In delivering judgment in that case, the chief judge said the issue of apostasy was related to Islamic law, and civil courts could not intervene.