Thousands of people gather in a downtown square in Serbia's Belgrade to light candles and lay flowers, mourning the 8 children and security guard that were killed during a school shooting. The incident rocked the Balkan nation, with the president calling it "one of the most difficult days" in Serbia's recent history. "We need to change something, so this never happens again," says Zdravko Spremo, father of two who came to pay his respects. IMAGES AND SOUNDBITES