A large explosion rocked an area of Iran’s capital where thousands were gathered for an annual state-organized rally to support the Palestinians and call for Israel’s demise. Israel had warned earlier Friday that it would operate in the area in central Tehran. There were no immediate reports of casualties. But the decision to proceed with the mass demonstrations, and Israel’s threat to target the area, underscored the fierce determination on both sides nearly two weeks into a war that has rattled the global economy and shows no sign of letting up.