A number of Western countries have backed claims of election fraud, and criticised Iran's handling of the anti-government demonstrators.
Angela Merkel, Germany's chancellor, joined her voice to the mix on Sunday, calling for a full recount of votes cast and for an end to the violence.
Mohsen Makhmalnaf, the foreign spokesman for the Iranian opposition leader Mir Hossein Mousavi, meanwhile, defended the actions of the protesters.
"These people are in the streets to say 'We don't want atomic bombs, we want democracy'," he told Al Jazeera from Paris.
On Saturday, about 3,000 opposition protesters had spilled on to Tehran's streets, undaunted by a warning from Ayatollah Ali Khamenei, Iran's supreme leader, not to continue demonstrations.
Security forces responded with live rounds, batons and tear gas, with the pandemonium continuing well into the night.
"The Iranian government must understand that the world is watching"
Witnesses said that dozens of people were hospitalised after being beaten by police and the pro-government Basiji militia.
Reports on community-driven websites such as Twitter claim a number of protesters were killed by police in the clashes.
One video uploaded to YouTube on Saturday alleged to show a teenage girl - being called Neda - dying on the street after being shot by police.
Al Jazeera was unable to verify the authenticity of the video or other reports of violence due to an official ban on independent reporting in the capital.
However, on blogs and social-networking websites, Neda was being held up as a symbol and a martyr for the protesters.
Including Saturday's toll, the Iranian government has admitted to the deaths of at least 20 people in unrest since the June 12 election.
State broadcaster IRINN said 100 people were injured Saturday's violence.
As the clashes took place, a suspected suicide bomber blew himself up outside the shrine of Ayatollah Ruhollah Khomeini, the leader of the Islamic revolution in 1979, injuring at least two people, local news agencies reported.
Government-run television said members of the exiled Mujahideen Khalq opposition group were arrested in connection with Saturday's unrest.
The report claimed they were acting under British influence.
Five family members of Ayatollah Hashemi Rafsanjani, a prominent Iranian politician and former president, were also reported to have been arrested, including his eldest daughter, for taking part in an illegal protest.
Mousavi, the defeated reformist candidate, and his supporters say the election was stolen by Ahmadinejad and has reiterated demands for the poll results to be annulled.
|Protests in Tehran were far smaller on Saturday than earlier rallies [AFP]
"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 and the idea that Islam is incompatible with a republic will be proven," he said in a statement posted on the website of his Kalemeh
Barack Obama, the US president, urged Tehran to allow Mousavi's supporters to stage peaceful protests and called for an end to the violence.
"The Iranian government must understand that the world is watching," he said.
"We mourn each and every innocent life that is lost. We call on the Iranian government to stop all violent and unjust actions against its own people."
Iran's government has repeatedly denied accusations of election fraud.
The contested result gave Ahmadinejad a tally of about 63 per cent, to Mousavi's 34 per cent.
Iran's highest legislative body, the Guardian Council, has offered a partial recount of ballots in order to appease protesters.
"Although there is no legal duty on us, we are ready to recount 10 per cent of the whole ballot boxes around the country randomly with presence of the respected representatives of the candidates," Abbas Ali Kadkhodaei, a spokesman for the council said on Saturday.
However, a partial recount is unlikely to end the more than a week of protests, with fresh calls being made for a general strike on Sunday.
"We want [a] revote, not [a] recount," Makhmalnaf told Al Jazeera.
He said Mousavi had been on the street alongside his supporters on Saturday night.
"We ask all people around the world not to confirm Ahmadinejad as our president," Makhmalnaf said.