By John Irish and Clotaire Achi PARIS (Reuters) – Thousands of anti-far right protesters marched across France on Saturday as opponents of presidential candidate Marine Le Pen seek to form a united front to prevent her from winning an election runoff against incumbent Emmanuel Macron on April 24. Macron, a pro-European Union centrist, won the presidency in 2017 after easily beating Le Pen when voters rallied behind him in the runoff to keep her far-right party out of power. This year, the first round of voting last Sunday set up the same battle, but Macron is facing a much tougher challenge. I…