By Pavel Polityuk and Valentyn Ogirenko KYIV (Reuters) – Several thousand Ukrainians rallied in Kyiv on Saturday to show unity amid fears of a Russian invasion, as Ukraine’s leader told people not to panic and pushed back against what he said was a glut of bleak war predictions being reported in the media. Tension has mounted as Russia has built up more than 100,000 troops near Ukraine and carried out large-scale exercises. The United States said on Friday an invasion could start at any moment. Russia denies planning to invade. Ukrainians filed through the centre of Kyiv in a column, chanting …