ISTANBUL — Hundreds of thousands of protesters turned out for a massive rally in Istanbul on Sunday evening, cheering the leader of the opposition as he concluded his three-week March for Justice and threw down a challenge to President Recep Tayyip Erdogan to institute changes or face a “revolt against injustice.”

“Nobody should think this march has ended; this march is a beginning,” Kemal Kilicdaroglu, the leader of the Republican People’s Party, known as C.H.P., said as he walked onto a stage to rippling cheers. “This is a rebirth for us, for our country and our children. We will revolt against injustice.”