Israel's warplanes, drones and helicopters appeared to shift focus from suspected Palestinian rocket sites to the northern Gaza frontier, where their bombs created incursion corridors by clearing landmines or guerrilla gun nests.
The mobilization was anything but secret and details put on social media by the Israeli military appeared to be a clear warning to the Hamas Islamists that govern Gaza to push for a truce.
"It is not our intention to go to war, and we are hopeful that this operation will not take a minute more than required," Israeli President Shimon Peres said.
Since being fought to a standstill in its 2006 war against Lebanese Hezbollah guerrillas, Israel has been honing the training of its regular troops and could mount a land invasion of Gaza at short notice.
Public statements by Prime Minister Benjamin Netanyahu suggest such an escalation has preliminary cabinet approval.
Among units already garrisoned outside Gaza is Israel's paratrooper brigade whose commander, Colonel Amir Baram, said last month that in planning tactics he had studied World War One skirmishes in Gaza between British forces and the Ottoman Turks.
Should his troops be ordered in, Defense Minister Ehud Barak told Channel 2 television, "they will need to go house-to-house, and then we will need the lessons of the past".Relief creates risk
Among those lessons learnt has been that Gaza's impoverished population of 1.7 million is vulnerable to humanitarian crises, which could spell international controversy for Israel.
Since the last Gaza war, of 2008-2009, the army says it has assigned some of its regiments with Arabic-fluent "relief officers" to direct Palestinian civilians away from danger.
To judge from the pace of the previous offensives in Lebanon and Gaza, it could take several more days for Israel to train and equip reservists for action.
The military declined to give details on where the reservists would serve, but Israeli media said they included personnel from homefront units that sound sirens during rocket attacks from Gaza and advise the public on where to shelter.
The reservists being called up on Friday were among a total of 30,000 whose draft was authorized by the Defense Ministry.
The scale of the potential mobilization prompted one commentator on Israel's Army Radio, who half-joked on air that so many troops would risk "falling over each other" in Gaza.
Interviewed by the station, Vice Premier Silvan Shalom was asked if Israel was preparing the military for possible flare-ups on other fronts, such as Lebanon, which Israel has watched with concern given conflict in neighboring Syria and furor over the nuclear ambitions of Hezbollah's patron Iran.
"We are taking everything into consideration," Shalom said, without elaborating.
Veteran commanders say around 30,000 troops altogether took part in the 2006 Lebanon war and 20,000 in the 2009 Gaza invasion. The number of garrisoned troops always stationed outside Gaza is a state secret.'Quiet for the south'
Yossi Peled, a recently retired cabinet minister from Prime Minister Benjamin Netanyahu's right-wing Likud party and a former army general, said Israel did not seek to topple Hamas or even to crush its outgunned guerrillas in "victory" battles.
"Quiet for the south (of Israel), that is the objective of the operation, writ big," Peled told public television, referring to years of sporadic rocket and mortar salvoes by Hamas and other Gaza factions that surged in the last two weeks.
Though at least 340 of the missiles have been fired since Wednesday's flare-up, the Israelis say they have made strategic gains by destroying, on the ground, around 20 Fajr rockets with 75 km ranges - capable of hitting deep inside Israel.
"They (Palestinians) may have a few left, but it is no longer the menace that it was," said one security official, even as Hamas and its allies managed to fire rockets at Tel Aviv and Jerusalem on Friday, causing no damage or casualties.
Both cities had previously been beyond the reach of Palestinian rocketry.
Israel's saturation air strikes - which peaked on Thursday, with a rate of one every five minutes, according to the chief military spokesman - have also razed a large number of suspected munitions factories and caches, the official said.
Another target has been fields where, the Israelis believe, Hamas and other Gaza factions positioned rocket "silos" - buried launch tubes, pointing across the border, that could be detonated remotely.
Those rockets that survived Israel's pre-emptive attacks and are fired have to get past Iron Dome, an air defense system that uses small radar-guided interceptor missiles.
Israel has four Iron Dome batteries deployed and its Defense Ministry said on Friday it had rushed forward production of a fifth so that it could be deployed as early as the weekend.
JERUSALEM (Reuters) -- Israeli tanks and troops massed outside Gaza and the military said on Friday it was calling up 16,000 reservists, signs of a possible imminent invasion of the Palestinian enclave after 48 hours of air strikes.