At least 51 people have been killed and 1,100 others injured during the past four days in violent, massive protests demanding the ouster of President Hosni Mubarak, the Health Ministry said Saturday.

The Cabinet resigned in the midst of rampant looting across the sprawling capital city dotted by dozens of military armored personnel carriers and tanks as well as soldiers on foot deployed around a number of key government buildings.

The pyramids on the outskirts of Cairo -- Egypt's premiere tourist site -- were closed by the military to tourists.

A police crackdown drew harsh criticism from the Obama administration and even a threat Friday to reduce a 1.5 billion U.S. dollar foreign aid program if Washington's most important Arab ally escalates the use of force.

Hundreds of protesters on Saturday resumed demostrations in Tahrir Square in downtown Cairo, where anti-government protests started peacefully on Tuesday before turning violent in the following days.

Witnesses said several tanks were parked near the square but no intervention in the protests has been seen so far.

On Friday, protesters burned down the ruling party's headquarters complex along the Nile in one of the more dramatic scenes in a day of utter chaos.

As the protests entered their fifth straight day, the military extended a curfew on Saturday in Cairo, Alexandria and Suez from 4:00 p.m. local time (1400 GMT) to 8:00 a.m. local time (0600 GMT) the following day, according to state television.

According to a statement released by the Egypt's state news agency MENA, the military earlier asked citizens to obey the curfew and refrain from gathering in large groups in public places.

"Legal measures will be taken against those in violation," the statement said.

Also on Saturday, Egypt's cabinet officially resigned during a meeting following Mubarak's call for them to step down, Nile television station reported.

In his TV address, Mubarak said a new cabinet would bring more democracy to the country in response to nationwide protests. But Mubarak, who has been in office for 30 years, refused to step down.

Demonstrators did not appear satisfied with Mubarak's actions to address the discontent and expressed doubt he will deliver on promised reforms.