Singapore authorities said Monday they had arrested three teenagers over separate plots to carry out shooting, stabbing and bomb attacks, warning of the threat of "youth radicalization" in the security-conscious city-state.

A 14-year-old boy created a 21-page manifesto detailing an attack on his schoolmates and teachers and admitted he would have carried out the stabbings.

The teens, aged 14 to 19, were held under the Internal Security Act, which allows for detention without trial.