Power went off in Gaza City last night after an Israeli blockade cut off fuel to the territory's only power plant.
After Hamas officials closed down the plant just before 8pm local time last night, Health Ministry official Dr Moaiya Hassanain warned that the fuel cut-off would cause a health catastrophe.
Four hours after the blackout, Hamas claimed that five patients died because of the power cut in hospitals, although this could not be confirmed.
In addition to the fuel it receives from Israel to power its electrical plant, Gaza gets most of its electricity directly from Israel. Israeli officials said that supply would not be affected.
Israel sealed all crossings into Gaza last week because of an increase in rocket barrages, cutting off fuel, food and medicine.
Last night, Palestinian President Mahmoud Abbas appealed to Israel to lift the blockade. Mr Abbas effectively rules only the West Bank after Hamas expelled his forces from Gaza last June.
Minutes after the power plant shutdown, Gaza residents started a candlelight march. The Hamas-linked Popular Resistance Committees threatened to break the Israeli blockade by crashing through the border with Egypt "by force."
Human rights groups condemned the fuel cut-off.
Israel pulled out of Gaza in 2005, but many see Israel as still responsible for the region, since it controls most land, sea and air access to the territory.
Last night Israel carried out two air strikes in Gaza City, Palestinian security sources said. One militant linked to Islamic Jihad was killed, it was reported.