Israeli intelligence officials say militant groups are planning a major "non-conventional" attack on New Year's Eve.
Possible targets are holy sites, nursery schools, apartment buildings and hospitals, they say.
Police have been told to prepare for three possible scenarios: an air- or sea-based attack or a ground assault involving several simultaneous suicide bombings, the officials said.
Senior intelligence officials know which of the three scenarios is most likely but are reluctant to share the information with police officers to prevent leaks, the officials said.
The warnings are connected to Israel's killing of a senior Islamic Jihad militant in the Gaza Strip on Thursday.
Following the airstrike, Defence Minister Shaul Mofaz said the death had prevented a "mega-terror attack," but security officials said the warnings are still relevant.
PA