Rarely do Egyptian demonstrations see thousands of people take to the streets. But, put together anti-Israeli and anti-government sentiments spearheaded by the powerful Muslim Brotherhood, and the result is the country's largest street action since the first anniversary of the American invasion of Iraq in 2004.

With Israel vowing a "long" siege against the Gaza Strip, there are concerns that the already strong opposition Brotherhood could become even stronger in the face of Israel's military action against Palestinians.

On Monday, thousands of Egyptians marched in downtown Cairo, chanting phrases such as, "Off to Gaza we go, martyrs by the million," and "We all belong to Hamas."

The government and the military were as much to blame as Israel, activists said. "Where is the Egyptian army?" was another slogan chanted by the throngs of demonstrators.

"It is disappointing that while our Palestinian brothers and sisters are being killed, [President Hosni] Mubarak and his cronies sit by and do nothing. Only the Brotherhood is leading this country on the right path," says Ahmed Said, a university student who is supporting the efforts of the Brotherhood.

Egypt is no stranger to massive street protests in the face of Israeli action. In 2006, when Israel began what turned into a month-long war against Hizbullah and Lebanon, the opposition, led by the Brotherhood, took to the streets to voice its support.

This year has already seen thousands more flood Cairo's streets in what have inevitably become anti-Mubarak protests, as well as anti-Israel.

Unlike two years ago when a sea of yellow could be seen in Cairo showing solidarity with Hizbullah and the Lebanese, this week has already seen much larger demonstrations waving Hamas flags.

While the Brotherhood has been leading efforts to demonstrate, those protesting are a mixture of the Egyptian opposition spectrum: secularist, Islamist, Communist and others, who believe Israeli military action is unjustified and hold the Egyptian government responsible for what is occurring in Gaza.

One day earlier, Brotherhood members of parliament presented a memo to Mubarak requesting Egypt's role "exceed transporting the injured." It added that Cairo must call for an Arab summit.

Assistant secretary-general of the MB Parliamentary Bloc, Mohamed El Beltagy, says Egypt must take the "first step toward unifying Arabs in ending the Gaza holocaust."

The demonstrations have echoed the memo's sentiments and shown the Brotherhood is able to arouse thousands of its supporters to action when it chooses.

"We are calling for the expulsion of the Israeli ambassador and ending the prevention of humanitarian relief caravans heading to Gaza and facilitating their efforts," adds Beltagy.

On Tuesday, the Brotherhood's Supreme Guide, Mohamed Mehdi Akef, urged Egyptians to continue their demonstrations in solidarity with Gaza, but told his supporters to avoid a violent confrontation with security forces. Akef himself marched with protestors on Monday.

"No doubt that your solidarity with the people of Palestine will support their firmness and encourage and inspire Gaza's heroes in their confrontation with Israel and journey to freedom, by the will of God," his statement read.

Being overlooked due to the fighting in Gaza is the power of the Brotherhood and its massive grassroots support. The ability to bring thousands of people to the street under one unified banner is extremely difficult in Egypt and the success this week seen by the Brotherhood is causing many to question the true strength of the group.

"It is great to see so many people out and supporting the cause of Palestine. It is unacceptable that our Arab governments sit by and do nothing while the people want action," one protestor said at Monday's demonstration, which turned into a silent march from downtown Cairo to the Doctor's Syndicate as part of an agreement with security forces.

With a government largely seen by the opposition as being complicit in the bombing of Gaza, Egyptians are fed up and heading to support the Brotherhood in droves. Pictures of Israeli Foreign Minister Tzipi Livni meeting with her Egyptian counterpart Ahmed Aboul Gheit late last week have fueled resentment and anger among average citizens.

Although chants of "Down with Mubarak" were largely silenced at Sunday's and Monday's demonstrations, the feelings remain. Protestors believe the government is as much to blame as Israel and the United States. And in a region where conservatism is taking hold, the outlet is the Brotherhood - the country's most organized and vocal opposition force.

"I don't normally support the Brotherhood and don't actually like it that much," says Heba, a self-proclaimed secular activist. "But in this situation, without any other actions that can be taken, I will support the Brotherhood and its continuous action against the government and Israel. It is so horrible what is happening in Gaza and our governments are doing nothing."

Her sentiments echo the mainstream these days, which accuses the government of doing nothing and saying nothing save a few veiled criticisms of Israeli action.

The Brotherhood has become the only force of leadership in Egypt, making it scarily powerful in the face of Mubarak's rule. However, violence is not an option, says Brotherhood deputy Mohamed Habib.

"We do not want violence. That is not the purpose of our demonstrations and mass movement by the people," he told The Media Line.

"Our goal is to spur the government to take action in what has become the most recent in a string of wrongs brought on Arabs by Israel."

The government is in a precarious situation. First, the peace treaty with Israel limits its ability to apply pressure; second is the alleged prior knowledge of Israeli plans by Cairo before the bombing of Gaza began on Saturday.

This, along with the widespread support the Brotherhood is achieving this week means the government is not in a position to do much.

With more demonstrations planned for this week and next, the Israeli attacks have given new life to a struggling opposition and the Brotherhood is taking the lead in formulating a response that has the majority falling into line.

As the death toll in Gaza continues to rise, the likelihood that Arab, and Egyptian, frustrations will abate is doubtful, leading some to question if Egyptian knowledge of such an attack and its inaction to stop it, could be the beginning of a new era in Egyptian politics and power.