BEIJING, May 19 (Xinhua) -- China on Monday began a three-day national mourning for the tens of thousands of people killed in a powerful earthquake which struck the country's southwest on May 12. At 4:58 a.m., the national flag at the Tian'anmen Square in downtown Beijing flew at half-mast after a complete flag-raising ceremony. About 2,600 people watched the flag-raising ceremony in the square. "I have been watching TV to know the disaster situation these days," said Yu Huilin, a 58-year-old retired teacher. Yu just arrived in Beijing by bus in the early hours on Monday from eastern Shandong Province. "It's really heartrending," she said. "But I see the quake-affected people have got help from so many people. I believe they can recover from the disaster soon rebuild their homes." All national flags will fly at half-mast at home and Chinese diplomatic missions abroad from Monday to Wednesday. Public recreational activities will be halted during the mourning period. At 2:28 p.m. Monday, Chinese citizens nationwide will stand in silence for three minutes to mourn for the victims, while air raid sirens and horns of automobiles, trains and ships will wail in grief. In the mourning period, condolence books will be opened in China'