Egyptian leaders have promised to support Gaza against Israeli attacks, as violence escalated for a third day.
President Mohammed Mursi said he would not leave Gaza on its own and condemned Israel's "blatant aggression", hours after his prime minister visited Gaza.
Israel mounted a huge attack overnight, targeting dozens of locations.
Sirens sounded in Jerusalem for the first time as militants said they had launched a rocket - hours after Tel Aviv was targeted with no casualties.
Twenty Palestinians and three Israelis have been killed since Wednesday.
Militants and civilians, including at least five children, were among the Palestinian dead, Palestinian officials said.
They include Hamas's military leader Ahmed Jabari, killed by an Israeli air strike on Wednesday following a surge in rocket attacks from Gaza.
Two Israeli women and a man died when a rocket fired from Gaza hit a building in the southern town of Kiryat Malachi on Thursday, Israeli officials said.
Before the recent upsurge, Israel had repeatedly carried out air strikes on Gaza, as Palestinian militants fired rockets across the border.
The Israeli army began an initial draft of 16,000 reservists on Friday, after the government authorised the call-up of 30,000.
Rumours have been swirling that a ground attack is imminent, but Israeli officials have said no decision has yet been made.
Israel targeted more than 130 locations in an overnight assault it said was aimed at knocking out rocket-firing facilities.
Sporadic Israeli air strikes have continued throughout Friday, and militants in Gaza have fired dozens of rockets.
Mr Mursi sent Prime Minister Hisham Qandil to the territory for a three-hour visit on Friday to show solidarity with the Palestinians.
"Egypt will not leave Gaza on its own, and what is happening is a blatant aggression against humanity," the president said shortly after Mr Qandil returned from Gaza.
Ties between Hamas and Egypt have strengthened since Mr Mursi's election earlier this year.
Hamas was formed as an offshoot of the Muslim Brotherhood, to which Mr Mursi belongs.
Panic in Tel Aviv
Meanwhile, the Israeli city of Tel Aviv, some 75km (45 miles) north of Gaza, came under attack for a second day.
A missile launched from Gaza sparked panic in the city and air-raid sirens once again sounded.
The missile appears to have done no harm, with police officials quoted variously as saying it had landed in the sea or in an unpopulated area.
It is the first time Tel Aviv has come under attack since the 1991 Gulf War.
Analysts say it is the first time Gaza militants have deployed such powerful missiles.
An Israeli military spokesman said 550 missiles had been fired into Israel since Wednesday - 184 had been intercepted by its radar-protective system, Iron Dome.
The Israelis had hit more than 600 targets in Gaza, the spokesman said.
Western leaders have appealed for both sides to stop the escalation in violence.
Britain and Germany both said Hamas bears the brunt of the blame and should stop firing rockets immediately.
Jabaliya refugee camp
A man surveys the rubble of a house in Jabaliya refugee camp