The world's grief and compassion returned to the tsunami-battered coastlines of the Indian Ocean on Monday, marking one year since giant waves crashed ashore in a dozen countries, laying waste to coastal communities and sweeping away at least 216,000 lives. Under a clear sky and before a gentle sea, survivors, friends and relatives of those who died commemorated those lost in one of the worst natural disasters the modern world has experienced. World leaders sent condolence messages. In Indonesia's Aceh province, which was closest to the earthquake that spawned the waves and bore the brunt of the disaster, President Susilo Bambang Yudhoyono sounded a tsunami warning siren to herald one minute's silence at the moment one year ago that the first wave struck. "It was under the same blue sky, exactly one year ago that mother earth unleashed her most destructive power upon us," Yudhoyono said. "The assault began with a massive earthquake but ... that was only a prelude to the horrific catastrophe to come." Similar periods of silence were observed at official ceremonies attended by thousands in Thailand and Sri Lanka, where flags were lowered to half-staff and bells rung in remembrance. Muslim, Christian, Buddhist and Hindu prayer services were being held across the tsunami zone.