Smallville actress Allison Mack has been arrested for her alleged role in notorious upstate sex slave cult NXIVM (pronounced Nexium), sources said.

Mack was last seen chasing after the cult's leader Keith Raniere on March 25 as police hauled him away from the villa in Mexico where they had been hiding out.

Raniere, who allegedly kept women as slaves and branded his initials into their skin, was arrested for sex trafficking and forced labour and brought back to the US.

The whole thing operated like an extortion plot, with the women forced to follow his directions after revealing "highly damaging information about themselves or family members," authorities charged.

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Raniere, 57, who lived outside Albany, received financing for his twisted operation from Clare Bronfman, an heir to the Seagram's liquor fortune, according to authorities. He was jailed pending an April 27 bail hearing.

Mack was expected to appear in a Brooklyn court later today.

Founded by Raniere in 1998, NXIVM offers "programs that provide the philosophical and practical foundation necessary to acquire and build the skills for success, it's website reads.

Around 160,000 joined the group led by Raniere, who is known within its circles as "Vanguard", including Mack, Battlestar Gallactica star Nicki Clyne and Dynasty star Catherine Oxenberg's daughter, India.

Mack was reportedly introduced to the self-help group around 2006 by her Smallville co-star Kristin Kreuk.

Kreuk, who portrayed the young ­Superman's love interest, Lana Lang, signed up, but got out before female acolytes began getting forcibly branded.

Mack played Clark Kent's starry-eyed blond pursuer, Chloe Sullivan.

Kruik put out a statement after Raniere's arrest saying she left the group five years ago, did not experience any nefarious activities and has denounced his alleged abuse of women.