The attacks have been targeting Gaza since Wednesday.
The retaliatory fire from the coastal strip has struck the heart of Tel Aviv. On Thursday, the Israeli Army said a projectile fired from Gaza struck Rishon LeTzion, some 15 kilometers (nine miles) southeast of Tel Aviv, but there were no injuries or damage.
The Israeli Minister for Military Affairs, Ehud Barak has given the green light for the military to call up 30,000 reservists.
The return fire has sounded off air raid sirens across the occupied territories for the first time in nearly two decades.
Earlier in the day, the democratically-elected Palestinian Prime Minister, Ismail Haniyeh vowed revenge on Israel, saying Palestinians will keep up resistance despite the deadly attacks.
The Israeli military frequently carries out airstrikes and other attacks on the Gaza Strip, saying the acts of aggression are being conducted for defensive purposes. However, in violation of international law, disproportionate force is always used and civilians are often killed or injured.
The attacks rage on while Israel keeps up its crippling blockade on Gaza, which it imposed on the enclave in 2007.