In an unprecedented series of attacks, unidentified arsonists yesterday planted explosive devices in four major churches in Athens, Piraeus and Thessaloniki, though only the Piraeus bomb detonated, causing damage but no injuries. A homemade bomb, comprising two gas canisters attached to a timer, went off inside the Aghia Triada Church in Piraeus shortly before 3 p.m. Earlier police bomb disposal experts destroyed a similar device found inside the Athens Cathedral. Police cordoned off the area around the cathedral, next to the capital's main tourist district, after receiving a warning from an anonymous caller who also contacted the press. An anonymous caller had also warned police about the bomb in Piraeus but officers arrived too late to avert the blast. Two homemade explosive devices, again connected to timers, were also defused by police in Thessaloniki, inside the central churches of Aghia Sofia and Aghios Dimitrios. A warning about a bomb at the Athens central appeals court complex yesterday turned out to be a hoax.