GRUENHEIDE, Germany—Tesla fans flocked to the carmaker’s new factory near Berlin on Saturday, where Chief Executive Elon Musk staged a county-fair style festival to celebrate the site’s imminent opening. Musk hopes in coming weeks to get the green light to start production at the site, which at its peak will produce 500,000 battery-electric vehicles (BEVs) a year—more than double Germany’s BEV production in 2020. While Tesla has said the site will bring Germany significantly closer to achieving its e-mobility goals, some locals and environmental groups are unhappy with Musk’s approach which they say flies in the face of German business culture. “I would tell him to stop building electrocars but rather fly to Mars,” said Manu Hoyer, who led a small protest of locals who oppose the project on environmental grounds, saying it will pollute drinking water. Tesla has also submitted plans to invest 5 billion euros ($5.8 billion) in …