The attack on the YMCA was carried out early Friday. Masked gunmen came to the YMCA compound just after midnight and threatened two guards with pistols, said one of the watchmen, Rizek Ajrami.

The attackers then set off what Ajrami described as two homemade explosive devices. One left a small crater in the floor of the library and tore holes into the walls and ceiling. Books were strewn on the floor, covered by chunks of plaster, wicker bookshelves were overturned and wires dangled from the ceiling.

Hamas police declined comment. Gaza's Islamic militant Hamas rulers have repeatedly pledged to protect Gaza's about 3,000 Christians, a tiny minority in a territory of 1.4 million people. Christian institutions have been targeted several times since Hamas won parliament elections in 2006, and a Christian activist was killed in October.

For decades, Christians and Muslims had a relatively peaceful, if sometimes uneasy relationship in Gaza. Friendships spanned the religious divide and Muslims and Christian would visit each other on holidays, though intermarriage and proselytizing were strictly taboo.

Christians say they fear more attacks and, in the long run, for their future in Gaza. The community has been hit hard by emigration in recent years, with many Christians seeking jobs and a better life abroad.

Sitting in the courtyard of the YMCA compound Friday, engineering student George Tarazi, 24, a Christian, said he had an offer to work in Dubai once he graduates. He said he would not leave if he had better work prospects in Gaza. Tarazi said he believed a small group of extremists carried out Friday's attack, and that he was worried the number of extremists in Gaza could grow.

YMCA director Issa Saba said it was too soon to speculate about suspects, and insisted that Muslim-Christian ties remain strong. He said members of the security forces had inspected the scene and promised to investigate.