The targeting of Jerusalem followed a second day of rocket attacks by Gaza militants on southern Israel, as the Israeli Defence Force began mobilising thousands of its reserve troops, in preparation for a potential ground invasion.
Tanks - which usually lead Israeli incursions - along with troops were reported to be gathering near the Gaza border.
The rockets fired at Jerusalem and other Israeli cities, including the commercial capital Tel Aviv, followed a ratcheting up of strikes on Gaza by the Israeli military.
Sources said that more than 600 targets in the Palestinian enclave had been hit by last night, up from 225 on Thursday evening.
Sirens sounded repeatedly across southern Israel as more than 70 rockets were aimed at Ashdod, Ashkelon and Beersheba after an overnight lull that ended just before seven o’clock in the morning.
Israeli Prime Minister Benjamin Netanyahu convened his advisers in the heavily-fortified Kirya government compound in Tel Aviv to discuss the unexpected entry of Israel’s most populated region into the area of conflict, which first came under fire on Thursday.
“We did not choose to escalate the situation in the south, it was a decision of the Hamas, Islamic Jihad and the terror groups,” Mr Netanyahu said. “We are ready to expand the operation as necessary in a significant manner.”
The Mayor of Tel Aviv, Ron Huldai, ordered all public bomb shelters opened for the first time since the 1991 Gulf War.