Rescuers in Colombia were making last-ditch efforts to locate earthquake survivors early Thursday, as the critical 72-hour window for rescue operations neared its end. The magnitude 7.4 quake, the strongest to hit Colombia this century, has killed 265 people, injured more than 3,500 and left almost 500 missing, according to local authorities. In cities such as Cali and Pereira, emergency crews worked through a third night with their bare hands, cranes and sniffer dogs, stopping not to rest but to listen for signs of life. Thursday morning will mark 72 hours since the quake, a period when survivors are considered most likely to be found alive beneath collapsed buildings. Cali resident Martha Perez said she felt "helpless" as two relatives were thought to still be trapped under rubble. "We will hold on to (hope) until the very end," Perez told AFP on Wednesday night. "As you can see, all of us here -- the entire city of Cali -- are hoping to find more survivors," volunteer rescuer Fernando Loaiza said. Cali Mayor Alejandro Eder said Wednesday that rescue efforts were "entering the final phase." Colombia's newly installed president Abelardo de la Espriella has declared the disaster an "economic emergency" and announced three days of national mourning. "We will do what is necessary," he said in a statement, vowing to "emerge again as a country and as a society." More than 11,000 dwellings have been destroyed and thousands of families have been rendered homeless, according to official figures. - Holding onto 'hope' - In Pereira, crews focused on a collapsed bakery where they believe a street vendor trapped since Monday's earthquake may still be alive beneath the rubble. Rescuers using equipment that detects vibrations said they had picked up possible signs of life from Pablo Loaiza, prompting anxious relatives to gather near the wreckage. "They tell us it could be him," said his 16-year-old niece, Sara Loaiza. "My heart is about to burst out of my chest." Relatives said the trapped man appeared to respond when rescuers asked him to knock twice, then three times from beneath the rubble, drawing applause from those gathered at the scene. "Every half hour we make everyone be quiet to see if we can find people alive," volunteer rescuer Johana Sanchez told AFP. "So far, three people have been pulled out alive and three dead." Many have already spent consecutive nights outdoors, either because their homes were damaged or because they feared aftershocks. In the town of Roldanillo, Yuliana Mendez picked through the wreckage of the beauty products store where she worked, one of many businesses damaged in the disaster. "It's like starting over from scratch," she said. In Pereira's Providencia neighborhood, residents hauled furniture and household items from damaged buildings. Among them was lawyer Ana Paulina Crosthwaite, whose apartment block was left unsafe by the quake. "We're in a state of shock," she told AFP. "We're not prepared to tell our children that we have to pack whatever we can and start a new life." Many Colombians have mobilized to help, bringing water, food and supplies to affected neighborhoods, while long lines formed outside blood donation centers. "It's about wanting to help your community, your city and your country," said blood donor Ferney Cabrera. The United States announced $15.5 million in emergency aid to Colombia, and the European Union pledged two million euros ($2.3 million).