Srećan Uskrs!
http://www.spc.rs/eng/happy_easter

the most common phrase spoken is:

Христос васкрсе (Hristos vaskrse) - Christ resurrected.
to which you will get the answer:
Ваистину васкрсе (Vaistina vaskrse) - truly resurrected.

A clip from a TV news programme on the subject of Easter: