CHONGQING, China—Another aftershock registering magnitude 5.1 on the Richter scale rattled Sichuan Province and the nearby city of Chongqing at 1:20  A.M. local time, sending thousands of nervous residents into the street. Thousands of people huddled in People's Hall Square in the heart of downtown, including government officials—and most buildings over three stories were evacuated.

Jittery residents were further rattled by a government warning that another aftershock—potentially as strong as magnitude 7 on the Richter scale—could come today. Mobile phone networks failed as people across the region jammed the airwaves trying to reach relatives.

No major injuries were reported but the aftershocks may continue for weeks, according to authorities. The rumblings are complicating relief efforts.

The situation is not helped by a shortage of food in some of the hardest hit areas, like Mianzhu City where residents scrambled for aid supplies. The devastating earthquake also threatens to inflate already rising food prices; Sichuan contributes as much as 7 percent of China's grain supplies in summer as well as 5 percent of vegetables.

China was in the second of three days of mourning today, again memorializing victims of the earthquake at exactly 2:28  P.M., the time that the first temblor hit.

Passengers traveling on a riverboat stood stock-still with heads bowed for three minutes as all the passing river traffic blared horns, resounding off the canyon walls of the Three Gorges region.