Protesters gather in Washington

Washington is preparing for a massive rally in support of gun control, one that organizers hope will prove that the country has reached an emotional tipping point on gun violence, with teenagers seizing the initiative and leading the demand for change.

The nation’s capital is generally nonchalant about protests, but Saturday’s gathering has prompted more attention and speculation than usual.

Estimates on crowd size are notoriously unreliable, but organizers are hoping to draw 500,000 protesters. More than 800 other concurrent marches are planned in cities across America and dozens of locations overseas.

Eric Cabanis / AFP / Getty Images French members of the Research and Intervention Brigade secure the area during a search operation at the Ozanam housing estate in Carcassonne, France, on Friday, following the fatal shooting by French security officers of an armed hostage taker....