Witnesses said hundreds or even thousands of people took to the streets of Tehran on Tuesday afternoon, although curbs on the international media mean it is difficult to verify reports from Iran independently.
Large numbers of riot police were deployed to meet them, and militia men in civilian clothes also mingled with the crowd, the BBC’s Jon Leyne reports.
In a new form of protest, activists were urged to turn off lights and domestic appliances at 2055 (1625 GMT).
They planned to switch on five minutes later appliances that consume large amounts of electricity, such as irons, toasters and microwave ovens.
Activist leaders hoped the resulting surge in demand could cause a power outage and cloak Tehran in darkness, allowing some the chance to protest on the streets.
The power protest is the latest in a series of efforts to continue demonstrating without breaking the law or risking arrest.
Following a government clampdown on protests shortly after the disputed election, many in Tehran took to their roofs after dark to shout Allahu Akbar (God is Great) in solidarity with the opposition
