A little light Googling indicates that the only New York City bridge on the East River that opened in 1903 was the Williamburg, on December 19th, so we'll say it's that. A group of men in a mixture of silk hats and derbies march across the bridge, looking around suspiciously. One of them is presumably Seth Low, whose tenure as a mayor of New York City was erased in civic memory by Tammany Hall. Presumably the crowd is composed of Brooklyn Republicans, keeping an eye out for Manhattan Democrats armed with brickbats, eggs and snowballs to knock those toppers off their heads.
One of the ways you can tell that a movie was shot at a real event instead of one specially staged for the camera is that everyone wears a hat, and some people are going to look at the camera. Here, the camera-watching is incited by a small boy carried in his mother's arms, followed by several of the politicians. Those cameramen might be Democrats.