Amateur video claims to show the latest confrontation in Tehran
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, Reuters news agency said.
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.
Mr Mousavi has issued statements opposing the election result, saying detention of protesters would not end opposition.
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.
12 June Presidential election saw incumbent Mahmoud Ahmadinejad re-elected with 63% of vote
Main challenger Mir Hossein Mousavi called for result to be annulled, alleging poll fraud
Mass street protests saw at least 20 people killed, hundreds arrested, and foreign media restricted
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.
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.
Since the announcement that Mr Ahmadinejad had won June's presidential election, Mr Mousavi - a reformist former prime minister of Iran - has received backing from several senior figures within the Iranian establishment.
Among them are two former presidents, Hashemi Rafsanjani and Mohammad Khatami.
On Monday Mr Khatami called for a referendum on the legitimacy of the government. Only the Supreme Leader, Ayatollah Ali Khamenei, can organise a referendum, and he has already declared the election result valid.
Mr Khatami, quoted on Iranian websites, said millions of Iranians had lost faith in the electoral process.
But Iran's powerful Revolutionary Guard, which answers to the Supreme Leader, has described the election as a turning point. The head of the Guards' political bureau said the election allowed the Guards to take power into their own hands.
Our correspondent says many Iranians will see the Revolutionary Guards' confidence as confirming their own fears that the outcome of the election was just a thinly-veiled military coup.
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