An international organisation today expressed grave concern over the arrest of human rights lawyer P Uthayakumar for allegedly threatening a police officer over the handling of an investigation concerning a death in custody.

The Geneva-based World Organisation Against Torture or Organisation Mondiale Contre le Torture (OMCT) said it suspected that he was detained in relation to the case and his political activities in Parti Reformasi Insan Malaysia.

OMCT said it was concerned that Uthayakumar, who is Prim pro tem secretary-general, may be subjected to ill-treatment.

"People being detained incommunicado in Malaysia run a higher risk of being subjected to such treatment," it said in a press statement.

"[We] therefore call on the Malaysian authorities to guarantee Uthayakumar's personal integrity and to immediately release him in the absence of valid legal charges," OCMT added.

Uthayakumar was released at 1.30 pm today at the Sepang police station in Selangor but was immediately re-arrested. He was later brought to the Sentul police station in Kuala Lumpur.