MONTREAL—Yellow and blue flags flew in cities across Canada on Thursday as Ukrainian Canadians organized demonstrations to denounce Russia’s attack on Ukraine and urged Ottawa to act swiftly to defend their homeland. In Montreal, several dozen protesters gathered in front of McGill University, waving signs with messages ranging from “I want to see my grandmother alive,” to the more militaristic “Arm Ukraine.” Larysa Grynko, who came to Canada from Ukraine in 2011, was one of several who helped unfurl a large banner in Ukrainian blue and yellow. Her voice catching, she said she was “devastated” by the events. “Words cannot describe the terror that we feel for families that stay in Ukraine,” she said. Grynko said she had spoken to her family, and they were safe for now and had confidence in the Ukrainian army. She switched briefly to Ukrainian to deliver a tearful message that she said meant, “Be …