Moscow police say a man helping construct the stage for a major protest rally has died after a column of sound equipment collapsed.

The accident at midday Monday came just a few hours before opponents of President Vladimir Putin were to gather for a demonstration. The rally marks the one-year anniversary of a protest on the eve of Putin's inauguration that ended in clashes between police and demonstrators.

About two dozen people face charges stemming from the clashes; most remain in jail a year after the protest.

The Interfax news agency said the man killed Monday was a 25-year-old opposition supporter.

A city safety official was quoted by the RIA Novosti news agency as saying investigators were inspecting the accident site at Bolotnaya Square, on an island near the Kremlin.