Iran's Interior Ministry declared as illegal a rally which supporters of defeated moderate presidential candidate Mirhossein Mousavi plan to hold in Tehran later on Monday, Reuters reported.
Unrest has rocked Tehran and other cities since the Interior Ministry released results on Saturday that showed hardline President Mahmoud Ahmadinejad had defeated Mousavi by a landslide in Friday's presidential election.
Mousavi has appealed to the Islamic Republic's top legislative body to annul the result because of what he alleges were irregularities, a charge the Interior Ministry and Ahmadinejad have dismissed.
The election result has disconcerted Western powers trying to induce the world's fifth-biggest oil exporter to curb its nuclear program. U.S. President Barack Obama had urged Iran's leadership "to unclench its fist" for a new start in ties.
On Sunday, Mousavi's supporters handed out leaflets calling for a rally in downtown Tehran on Monday afternoon. The protests over the last two days are the sharpest show of discontent against the Islamic Republic's leadership for years.
"The Interior Ministry issued a statement and said no permission had been issued for a rally ... The holding of such a gathering would be illegal," state radio said.
"Some seditious elements had planned to hold a rally and by fabrication said they had permission from the Interior Ministry. Any disrupter of public security would be dealt with according to the law," it said.
State television said Ahmadinejad was due to fly to Russia later on Monday to attend a summit meeting, a day after holding a triumphant rally attended by a cheering crowd of tens of thousands of people.
Pro-Mousavi demonstrators threw stones at police at Tehran University on Sunday and also clashed with Ahmadinejad supporters on a main street in the city that was littered with broken glass and fires.
In the north of the capital, a stronghold of Mousavi backers, riot police patrolled after midnight. Rubbish burned in the street, some cars had their windows broken, and police blocked access to roads.
Mousavi's appeal
Supreme Leader Ayatollah Ali Khamenei has called on Iranians to support Ahmadinejad and state media quoted him as saying the unprecedented high election turnout was "a divine wonder."
In a statement on his website, Mousavi said he had asked Iran's legislative Guardian Council to cancel the vote result.
"I urge you, Iranian nation, to continue your nationwide protests in a peaceful and legal way," he said.
After dusk on Sunday some Mousavi supporters took to rooftops across Tehran calling out "Allah Akbar" (God is greatest), an echo of tactics by protesters in the 1979 Islamic revolution.
"It sure looks like the way they're suppressing speech, the way they're suppressing crowds, the way in which people are being treated, that there's some real doubt," he told NBC's "Meet the Press" when asked if Ahmadinejad had won the vote.
Germany, one of Iran's biggest trading partners and a negotiator in the West's nuclear talks with Tehran, said it had summoned the Iranian ambassador.
"We are looking toward Tehran with great concern at the moment. There are a lot of reports about electoral fraud," Foreign Minister Frank-Walter Steinmeier told German ZDF TV.
An adviser to French President Nicolas Sarkozy said what was happening in Iran was "clearly not good news for anyone, neither for the Iranians nor for peace and stability in the world."
Iran's refusal to halt atomic work the West suspects is aimed at making bombs, a charge Tehran denies, has sparked talk of possible U.S. or Israeli strikes on its nuclear sites.