Tehran was tense on Sunday after thousands clashed with police, defying an ultimatum from Iran's supreme leader Ayatollah Ali Khamenei for an end to protests over disputed presidential elections.
At least one person was wounded when shots rang out, a witness said after Saturday's rally which, like other opposition protests over the past week, was off-limits to the foreign media.
A suicide bomber meanwhile struck a key regime monument -- the south Tehran mausoleum of revolutionary leader Ayatollah Ruhollah Khomeini -- killing himself and wounding three people, two of them foreigners, state media reported.
The demonstrators braved tear gas and water cannon to assemble in Enghelab Square in the heart of the capital, witnesses said.
"The robocops beat us up badly," one protestor said. "Men and women were beaten up.... My whole body is bruised.... They confiscated my camera."
Another witness said: "Lots of guards on motorbikes closed in on us and beat us brutally.
"As we were running away the Basiji were waiting in side alleys with batons, but people opened their doors to us trapped in alleys."
Unlike on Friday, when Khamenei delivered a nationally broadcast address in support of the contested re-election of hardline incumbent Mahmoud Ahmadinejad, foreign news outlets were barred from covering the opposition protest.
Members of the Basij Islamic militia, which has been at the forefront of the regime's efforts to face down a week of protests against official results giving Ahmadinejad a new four-year term, were again out in force.
One of the rally organisers, the Combatant Clerics Assembly, had announced it was backing out after the authorities refused to grant authorisation but supporters of Ahmadinejad's defeated challengers turned out regardless.
On Friday, Khamenei demanded an end to the protests, warning that opposition politicians would be held responsible for any new bloodshed beyond the seven deaths already reported by state media.
The head of Iran's security council, Abbas Mohtaj, sent a specific warning to Ahmadinejad's main challenger, moderate former premier Mir Hossein Mousavi.
"Your national duty tells you to refrain from provoking illegal gatherings," he wrote in the letter made public on Saturday.
"Should you provoke and call for these illegal rallies you will be responsible for the consequences."
But Mousavi hit back, accusing the authorities of deforming the political system established by the Islamic revolution of 1979 through electoral "cheating."
"If this huge volume of cheating and changing the votes... which has hurt people's trust is presented as the very evidence of the lack of cheating, then it will butcher the republican aspect of the system," he said.
"If the people's trust is not matched by protecting their votes or if they are not able to defend their rights in a civil peaceful reaction, there will be dangerous ways ahead," he warned.
His strong criticism of the regime's conduct of the June 12 election was briefly removed from his newspaper website Kalemeh, but later reposted.
Electoral watchdog the Guardians Council said it was ready to randomly recount up to 10 percent of ballot boxes from the election, state television reported.
It made its offer after meeting to study the 646 complaints of poll violations registered by the three defeated candidates -- Mousavi, ex-parliament speaker Mehdi Karroubi and former Revolutionary Guards chief Mohsen Rezai.
The Iranian capital has been rocked by daily demonstrations since the re-election of Ahmadinejad drew accusations by his defeated challengers of massive vote fraud.
Siding with Ahmadinejad in his first public appearance since the vote, Iran's supreme leader on Friday ruled out major fraud.
"The people have chosen whom they wanted," Khamenei said.
He said there could be no doubting Ahmadinejad's re-election. "The legal mechanisms in our country do not allow cheating. How can one cheat with a margin of 11 million votes?" he asked.
US President Barack Obama warned Iran that the "world is watching" its actions.
"We call on the Iranian government to stop all violent and unjust actions against its own people," Obama said.
"The Iranian government must understand that the world is watching. We mourn each and every innocent life that is lost."
Green-clad protesters massed in Los Angeles, New York and at the gates of the White House in Washington on Saturday.
Protesters expressed their solidarity with their Iran-based counterparts and called on the United States and Obama to "Stand with Iran," as one placard put it.
In the face of the regime's biggest crisis since the 1979 overthrow of the pro-Western shah, Iran's Islamic rulers have repeatedly lashed out at "meddling" by foreign powers.
But Western governments insist they are merely trying to defend universal human rights, not interfere in Iran's internal affairs.