CAIRO — Cairo residents boarded up homes and set up neighborhood watches of citizens armed with guns, clubs and knives as looting and violence engulfed the capital.With the police having disappeared from the streets, the army expanded its presence of tanks and armored personnel carriers but mainly around government buildings. As night fell Saturday and the chaos continued, the military fanned out to neighborhoods across the city in a bid to quell the lawlessness.Residents reported gangs of youths, some on motorbikes, roaming the streets, looting supermarkets, shopping malls and stores. Some of the gangs made it to affluent residential areas in the suburbs, breaking into luxury homes and apartments. The crackle of gunfire could be heard in the city center as well as outlying districts.The tens of thousands of protesters who have thrown Egypt's 30-year-old regime into tumult come from all walks of life — conservative Muslims and Christians, yuppies and the unemployed, young and old. For many, the protests demanding that President Hosni Mubarak step down were a catalyst for years or decades of repressed anger at mistreatment at the hands of the state."The army guards big institutions like the gardens and museum, not people and their property," said Mighaz Sawzi, who lives in central Cairo. "God has to protect us, because who's going to protect us now?"