Emma González spoke for just under two minutes on Saturday before tens of thousands of demonstrators at the March for Our Lives rally in Washington, describing the effects of gun violence in emotional detail and reciting the names of classmates who had been killed.

Then she said nothing for four minutes and 26 seconds.

Ms. González, a senior at Marjory Stoneman Douglas High School in Parkland, Fla., has emerged as one of the most prominent faces among the student activists who have mobilized against gun violence after a shooting at their school last month that left 17 dead.

Their facility with social media has added urgency to demands for more gun control. Ms. González, who has more than 1.3 million followers on Twitter, has spent much of the last month urging her audience to turn out for Saturday’s marches.