* Egypt's Islamist leader faces dicey balancing act
* Egypt PM visits Gaza in show of solidarity
* Protesters plan to mass in Cairo's Tahrir Square
By Shaimaa Fayed
CAIRO, Nov 16 (Reuters) - Thousands of people protested in Egyptian cities on Friday against Israeli air strikes on Gaza and Egypt's president pledged to support the Palestinian enclave's population in the face of "blatant aggression".
Western governments are watching Egypt's response to the Gaza conflagration for signs of a more assertive stance towards Israel since an Islamist came to power in the Arab world's most populous nation.
President Mohamed Mursi is mindful of anti-Israeli sentiment among Egyptians emboldened by last year's Arab Spring uprising but needs to show Western allies his new government is no threat to Middle East peace.
His prime minister, Hisham Kandil, visited Gaza on Friday in a demonstration of solidarity after two days of strikes by Israeli warplanes targeting Gaza militants, who had stepped up rocket fire into Israel in recent weeks.
Gaza officials said 28 Palestinians, 16 of them civilians, had been killed in the enclave since Israel began the air offensive against the tiny, densely populated enclave ruled by the Islamist Hamas movement.
Three Israelis were killed by a rocket on Thursday.
"We see what is happening in Gaza as blatant aggression against humanity," Mursi said in comments carried by Egypt's state news agency. "I warn and repeat my warning to the aggressors that they will never rule over the people of Gaza.
"I tell them in the name of all the Egyptian people that Egypt today is not the Egypt of yesterday, and Arabs today are not the Arabs of yesterday."
The Egyptian foreign minister also spoke to his counterparts in the United States, Jordan, Brazil and Italy on Friday to discuss the situation in Gaza, a ministry statement said.
Mohamed Kamel Amr spoke to U.S. Secretary of State Hillary Clinton about the necessity of cooperation between the United States and Egypt to end the military confrontations. Amr stressed the necessity of Israel ending attacks on Gaza and a truce being rebuilt between the two sides, the statement said.
Israeli ministers were asked to endorse the call-up of up to 75,000 reservists after Gaza militants nearly hit Jerusalem with a rocket for the first time in decades and fired at Tel Aviv for a second day. Such a call-up could be the precursor of a ground invasion into Gaza, or just psychological warfare.
Mursi's toppled predecessor, Hosni Mubarak, was a staunch U.S. ally who upheld a cold but stable peace with Israel.
The new president has vowed to respect the 1979 peace treaty with the Jewish state. But relations have been strained by protests that forced the evacuation of Israel's ambassador to Cairo last year and cross-border attacks by Islamist militants.
More than 1,000 people gathered near Cairo's al-Azhar mosque after Friday prayers, many waving Egyptian and Palestinian flags.
"Gaza Gaza, symbol of pride", they chanted, and "generation after generation, we declare our enmity towards you, Israel".
"I cannot, as an Egyptian, an Arab and a Muslim, just sit back and watch the massacres in Gaza," said protester Abdel Aziz Nagy, 25, a member of the Muslim Brotherhood.
Protesters were marching from other areas of Cairo towards Tahrir Square, the main rallying point for last year's uprising that toppled Mubarak.
In Alexandria, around 2,000 protesters gathered in front of a mosque, some holding posters demanding Egypt's border crossing to Gaza be opened to allow aid into the impoverished enclave.
Hundreds also gathered in the cities of Ismailia, Suez and al-Arish to denounce Israel's attacks.
Al-Azhar, Egypt's influential seat of Islamic learning, called on all Arabs and Muslims to unite in support of their brothers in Gaza, the state news agency MENA said.
"The Zionists are seeking to eliminate all (Palestinians) in the West Bank and the Gaza Strip," Ahmed al-Tayyib, the Grand Imam of al-Azhar, said in comments carried by MENA.
Al-Tayyib denounced the position of world powers on the Gaza crisis, describing them as having "forgotten their humanitarian duties ... and standing on the side of the aggressors," according to MENA.