Indian Prime Minister Manmohan Singh urged calm and vowed to protect women as police on Monday struggled to quell increasing outrage over sex crimes following the gang-rape of a student.

"There is genuine and justified anger and anguish at this ghastly incident," Singh said in a statement issued on Sunday as he appealed for "peace and calm" after demonstrations in the wake of the attack in New Delhi.

"I assure you that we will make all possible efforts to ensure security and safety to all women in this country."