Gunmen have attacked the Young Men's Christian Association (YMCA) in Gaza City and blown up its library, burning thousands of books, its director says.
There has been no claim of responsibility for the YMCA attack
Eissa Saba said 14 men overpowered the centre's two security guards before placing bombs in the library and main office. The latter did not explode.
The guards said the gunmen had asked them why they worked for "infidels".
Gaza, which is controlled by the Islamist group Hamas, is home to 3,500 Christians, mostly Greek Orthodox.
A number of Christian and other institutions regarded by Muslim extremists as un-Islamic have been targeted by armed gangs over the past two years in the coastal territory, the BBC's Katya Adler in Jerusalem says.
So far no-one has claimed responsibility for the attack, which has been condemned by all the main Palestinian factions.
Hamas, which ousted the rival Fatah movement from the territory in June, has said it is looking into Friday's incident.
The YMCA in Gaza City is a social institution open to Palestinians from all communities.