Iranian riot police are reported to have arrested a number of pro-reform protesters in Tehran after demonstrations turned violent.
Police clashed with hundreds of people marching despite a ban on public gatherings since the disputed election in June.
The re-election of President Mahmoud Ahmadinejad sparked widespread protests and allegations of vote-rigging.
Defeated candidate Mir Hossein Mousavi has continued to contest the result.
Witnesses said hundreds or even thousands of people took to the streets of Tehran, although curbs on the international media mean it is difficult to verify reports from Iran independently.
The BBC reports that large numbers of riot police were deployed to meet them, and militia men in civilian clothes also mingled with the crowd.
In a new form of protest, activists were urged to turn off lights and domestic appliances.
Five minutes later, they planned to switch on 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 renewed street protests come shortly after Mr Mousavi issued one of his strongest critiques of Iran's ruling elite since the election result was declared.
He said it was an insult to the nation to suggest that opposition members were only protesting because they were told to by foreign powers.
And he insisted that intimidation and threats would not silence his supporters.