CHICAGO — Cook County prosecutors dropped charges Wednesday against a protest leader arrested in the Loop hours after the release of the video showing a Chicago police officer fatally shooting 17-year-old Laquan McDonald. Malcolm London, 22, was charged with aggravated battery to a police officer, a felony. He struck an officer at a protest rally, according to the Chicago Police Department. Wearing a black T-shirt that read “apologetically black” on the back, London thanked the judge and smiled at supporters who packed a courtroom at the Leighton Criminal Court Building after a prosecutor announced the charges had been dropped. Supporters cheered and clapped as they left the courthouse and deputies yelled at them to quiet down. London had faced the most serious charge of those arrested during the demonstration, which lasted for hours and briefly stopped traffic on the Eisenhower Expressway. Police said in a statement that London “was identified as striking an on-duty Chicago police officer” as police blocked protesters from marching across the bridge. Supporters created a Free Malcolm London hashtag and urged people to call police to demand that he be released. “He was just standing there and the police snatched him up,” tweeted the Black Youth Project 100, which organized Tuesday night’s march.