CILACAP, Indonesia - Three Islamist militants convicted over the Bali nightclub bombings that killed 202 people awaited execution Sunday as Indonesia stood guard against a feared extremist backlash.

A source at the Nusakambangan island prison off southern Java said Amrozi, 47, his brother Mukhlas, 48, and attack strategist Imam Samudra, 38, had been placed in isolation and the execution order had been delivered.

Residents at Amrozi's and Mukhlas's home village of Tenggulun, East Java, woke Saturday to find an "H" had been painted on the ground overnight in a nearby field, where a helicopter from Nusakambangan might land with the bodies.

Local television also reported that checkpoints had been set up around Samudra's home town of Serang, and that a grave had been prepared.

Security forces have been placed on high alert across the mainly Muslim country as a precaution against an explosion of Islamist anger at the first executions to be carried out under Indonesia's anti-terror law.

Areas like foreign embassies, tourist spots and shopping malls were under close guard.

Hand-written posters appeared overnight in East Java promising retaliation for the bombers' execution.

The 2002 bombings targeted nightspots packed with Western tourists, killing more than 160 foreigners.