MOSCOW — Thousands of opponents of President Vladimir Putin recently crowded into a Moscow square infamous as the location of the state security forces, vowing not to let up pressure on their government after a year of protests that started with grand hopes for change in their country and ended far more tentatively.

They braved near-zero temperatures and official warnings against the gathering to hold an illegal protest in front of the imposing security headquarters. Many of the roughly 3,000 protesters laid flowers near a stone that commemorates victims of Soviet gulags in honor of what they say are modern-day political prisoners.