KUALA LUMPUR (Reuters) - The leader of Malaysian pro-democracy group Bersih and critic of Prime Minister Najib Razak was released on Monday after ten days in detention under a widely criticized security law.

Maria Chin Abdullah was arrested the day before a protest rally organized by the group saw thousands of Malaysians marching in the capital to demand Najib's resignation over his alleged involvement in a multi-billion dollar financial scandal. Najib has denied wrongdoing.

Maria was arrested on Nov 18 under Malaysia's Security Offences (Special Measures) Act, a law that was introduced in 2012 to fight security and extremist threats.

The U.S. State Department and other civil rights groups had criticized Maria's arrest under the law, which could have seen her detained for 28 days without trial.

In a text message to Reuters, human rights lawyer Ambiga Sreenevasan confirmed Maria's release, but said police had not given a reason why she was freed.

Earlier on Monday, police also raided the office of Empower Malaysia, a women's rights organization where Maria was previously executive director.

Empower Malaysia member Honey Tan said via her Twitter account lawyers were barred by police from entering the organization's office during the raid.

A police spokeswoman did not immediately respond to a request for comment.

(Reporting by Rozanna Latiff; Editing by Nick Macfie)