Mogadishu - Maryan Abdullahi's father had been preparing to fly to Somalia's capital on Sunday to attend the happiest moment in his daughter's life: her graduation day.Instead, he will bury her.On Saturday, the 22-year-old Abdullahi was making final preparations for graduation after years of medical studies when a massive truck bomb detonated in the middle of a Mogadishu street where she was travelling in a car with friends.They were killed in what has been called the single deadliest attack in Somalia's history. At least 189 people are dead, police and hospital workers said on Sunday, and more than 200 others were injured."A sad day. Her family was so proud of her and were counting the hours before her graduation," said Amino Ahmed, Abdullahi's classmate, with tears in her reddened eyes. "The life of a rising star was sadly cut short."As stories of the victims emerged, bewildered family members were still picking their way through the rubble on Sunday. Ambulance sirens wailed, and overwhelmed hospitals pleaded for blood donations.Mogadishu, a city long accustomed to deadly bombings by the Somalia-based al-Shabaab Islamic extremist group, was stunned by the force of Saturday's blast. As the huge plume of smoke began to rise, residents said the explosion was the most powerful they'd heard in years."With a heavy heart I would say this is the worst attack I have ever witnessed," said Masoud Mohamed, who lost family members in the blast.Parts of the busy Hodan district were flattened, with concrete buildings mangled. Cars burned. Bodies were rushed away by shaken survivors, with bloodied sandals left behind. Children carried away charred remains in cardboard boxes.Three brothers working at a nearby shopping centre were among the dead, witnesses said.The explosion also killed a couple who were returning from a hospital after having their first child, said Dahir Amin Jesow, a Somali lawmaker."It's a dark day for us," he said.Dozens of weeping women stood outside hospitals, while Mogadishu's mayor sent a desperate plea to residents to donate blood for the many wounded.