MOSCOW — About 20,000 people marched through Moscow on Sunday to protest Russia’s new law banning Americans from adopting Russian children, a far bigger number than expected in a sign that outrage over the ban has breathed some life into the dispirited anti-Kremlin opposition movement.

Shouting ‘‘shame on the scum,’’ protesters carried posters of President Vladimir Putin and members of Russia’s Parliament who overwhelmingly voted for the law last month.