Witnesses told The Associated Press that some protesters fought back, chanting: "Where is my vote?"
They said others described scenes of brutality - including the alleged police beating of an elderly woman - in the clashes around the Ghoba Mosque.
The reports could not immediately be independently verified because of tight restrictions imposed on journalists in Iran.
North Tehran is a base of support for opposition leader Mir Hossein Mousavi, who has alleged massive fraud in Iran's disputed June 12 presidential election and insists he - not President Mahmoud Ahmadinejad - is the rightful winner.
Sunday's clashes broke out at a rally that had been planned to coincide with a memorial held each year for Ayatollah Mohammad Beheshti, who came to be considered a martyr in the Islamic Republic after he was killed in a 1981 anti-regime bombing.
It was Iran's first election-related unrest since Wednesday, when a small group of rock-throwing protesters who had gathered near parliament was quickly overwhelmed by police forces using tear gas and clubs.
Iran's standoff with the West over its crackdown on opposition protesters escalated Sunday after authorities detained several local employees of the British Embassy in Tehran - a move that Britain's foreign secretary called "harassment and intimidation." The European Union condemned the arrests.
, but gave no further details. British Foreign Secretary David Miliband said "about nine" employees were detained Saturday and that four had been released.
EU foreign ministers meeting in Corfu, Greece, issued a statement Sunday condemning the arrests and calling for the immediate release of all those still detained. The 27-nation bloc also denounced Iran's continuing restrictions on journalists.
"They make clear to the Iranian authorities that harassment or intimidation of foreign or Iranian staff working in embassies will be met with a strong and collective EU response," the statement said.
Iran has accused the West of stoking unrest, singling out Britain and the U.S. for alleged meddling and for expressing concern about the ferocity of the regime's crackdown on protesters. Last week, Iran expelled two British diplomats, and Britain responded in kind. Iran has also said it's considering downgrading diplomatic ties with Britain.
On Sunday, the semiofficial Fars news agency reported that the embassy staffers were detained for what was described as a "significant role" in postelection unrest.
The British Foreign Office says the embassy has a staff of more than 100, including at least 70 locally hired Iranians. Last week, Britain sent home 12 dependents of embassy staff because the protests had disrupted their lives.
Miliband, in Corfu for the EU meeting, said Britain lodged a protest with the Iranian authorities over the detentions. He described the step as "harassment and intimidation of a kind that is quite unacceptable."
"The idea that the British Embassy is somehow behind the demonstrations and protests that have been taking place in Tehran. ... is wholly without foundation," he said.
Iran's government has tried to discredit opposition supporters by alleging they have been directed by the West.
On Friday, a senior Iranian cleric, Ahmed Khatami, lashed out at Britain in a nationally televised sermon. "In this unrest, Britons have behaved very mischievously and it is fair to add the slogan of 'Down with England' to the slogan of 'Down with U.S.A.,'" he said.
Britain, a former colonial power in the region with a long history in Iran, has been a prominent target. Britain and the U.S. were behind the 1953 coup that toppled Prime Minster Mohammad Mossadegh, who nationalized Iran's oil industry. Britain had almost complete control over Iran's oil industry for decades.
The British have also drawn fire because of the BBC's prominent role as a trusted broadcaster in Farsi inside Iran.
This is a reversal from the way the state and publicly funded BBC was perceived in the run-up to the Iranian Islamic Revolution. At the time, the BBC was widely listened to because it extensively covered anti-Shah demonstrations and activities of the Islamic Republic's founder, Ayatollah Ruhollah Khomeini, who was in exile in France.
Iran's leaders have countered Western condemnation with increasingly angry rhetoric. The confrontation appears to be dashing hopes for a new dialogue, as initially envisioned by President Barack Obama when he took office.
Mr. Obama wants to engage Iranian leaders in talks over the country's suspect nuclear program which the U.S. and other western countries worry is aimed at developing nuclear weapons. Iran defends its nuclear program as civilian in nature. On Sunday, EU foreign policy chief Javier Solana said the bloc would "like very much" to restart nuclear talks with Tehran despite the rising tensions.
Senior White House adviser David Axelrod played down Ahmadinejad's accusations against the U.S., saying Sunday they aren't credible and are meant for domestic consumption. "This is political theater," he said on ABC's "This Week."
Iran's rulers have unleashed club-wielding militiamen to crush street protests and arrested hundreds of journalists, students and activists.
On Sunday, Iran's supreme leader, Ayatollah Ali Khamenei, called for national unity, appealing to both sides in the dispute, even though he has come down firmly on the side of Ahmadinejad.
"I admonish both sides not to stoke the emotions of the young or pit the people against each other," he said in comments carried on state TV. "Our people are made of one fabric."
Mousavi signaled he is not dropping his political challenge.
In a new statement, he insisted on a repeat of the election and rejected a partial recount being proposed by the government. However, Mousavi's challenge seemed largely aimed at maintaining some role as an opposition figure.
The latest statement by Mousavi, who has been increasingly isolated, appeared Sunday on Ghalamnews, a Web site run by supporters. Mousavi-related Web sites have frequently been blocked by the government, and one was shut down by hackers last week.