MOSCOW - In the biggest show of opposition to Russian President Vladimir V. Putin since last May, thousands of demonstrators gathered Monday near the Kremlin to demand an immediate release of all political prisoners and new presidential and parliamentary elections.

The protesters filled Bolotnaya Square, not far from the site of a mass protest march exactly one year earlier. That demonstration, on the eve of Putin's third presidential inauguration, ended violently, in clashes with police.

"We have spent a year in deep defense," opposition leader Alexei Yashin told the crowd Monday. "It is high time we shift to a counteroffensive, and we will win back Moscow and then the entire country!"

This time, the rally ended peacefully, if ominously. With police helicopters hovering low under leaden skies, protesters scattered to find thousands of riot police and Interior Ministry troops lining every adjacent street and surrounding Red Square and the Kremlin.

In the year between the two gatherings, the Kremlin has been steadily cracking down on the opposition. Twenty-seven political activists, 12 of them in jail, are under investigation for allegedly organizing riots funded from abroad.

In addition, prosecutors have brought embezzlement charges against the most popular and charismatic of the opposition leaders, Alexei Navalny, who nonetheless appeared at the rally, arriving to massive cheers and applause as he climbed onstage with his wife, Yulia. In a fiery speech, Navalny gave the crowd a new rallying cry by calling Putin "a thieving, corrupt hypocrite" - a phrase that was immediately taken up and chanted.

If found guilty on charges of embezzling about $500,000 from an obscure regional timber company, Navalny could go to prison for up to 10 years and would become Russia's second most famous political prisoner after the former oil billionaire Mikhail Khodorkovsky. Navalny has called the charges against him politically motivated and false.

The day began badly. A few hours before the rally, a large loudspeaker fell on a young technician setting up audio equipment, killing him. Stunned organizers considered canceling the event but ultimately decided to press forward. The rally started with a minute of silence for the dead worker.