Gaza City was plunged into darkness after nightfall Sunday when Hamas officials shut down the territory's only electricity plant, following a cutoff of fuel from Israel. Israel blockaded Gaza Thursday as a pressure tactic against gunmen who have been firing rockets at Israel every day. The stricken power plant generates about one third of Gaza's electricity. The rest, which comes from Israel, was not affected by the blockade, Israeli officials said. Health officials and human rights groups warned of a humanitarian crisis in Gaza as a result of the Israeli blockade.