JAKARTA, Indonesia — A suicide bomber blew himself up as police were praying today, wounding 28 people in the first attack on a mosque since extremists started targeting the predominantly Muslim country a decade ago.

The victims — including a local police chief — were rushed to hospitals with nails, nuts and bolts embedded in their bodies, said Yeni Rahmawati, a hospital spokeswoman. The attack occurred in the West Java town of Cirebon.

Though houses of worship are commonly targeted by militants in Afghanistan, Pakistan and Iraq, this was a first for Indonesia, and experts worry it could signal a “hardening” of local militants.

Indonesia, a secular nation of 237 million people, was thrust into the front lines in the battle against terrorism in 2002, when the al-Qaida-linked regional network Jemaah Islamiyah attacked two crowded nightclubs on Bali island, killing 202 people, most of them foreign tourists.

The mosque stood on the grounds of a police compound, but was open to the public. Most of the wounded were officers.