Protesters chanting "God is great" grew angrier in Tehran and other cities Sunday as incumbent President Mahmoud Ahmadinejad insisted that he had won re-election fairly and police arrested more than 100 opposition supporters.
The regime mobilized thousands of Ahmadinejad supporters for a counterdemonstration in central Tehran's Vali Asr Square -- the scene of anti-Ahmadinejad riots Saturday -- to cheer the president and lambaste his opponents, the West and its media.
"We don't want an American [backed] revolution," demonstrators shouted as they walked down a traffic overpass to join a sea of Ahmadinejad supporters waving red, green and white Iranian flags.
"Death to those against the Spiritual Leader" was another slogan, alongside "Mousavi bye-bye," in a taunting retort to supporters of Mir Hossein Mousavi who had shouted "Mahmoud bye-bye" a few days before the Friday elections.
The "spiritual leader" remark was a reference to Ayatollah Ali Khamenei, the country's paramount official. He endorsed Mr. Ahmadinejad's re-election through a statement read on television but has not appeared before the public since the disputed vote.
Mr. Ahmadinejad earlier Sunday dismissed complaints by Mr. Mousavi of massive electoral fraud, and he compared street protests over the results to "problems at a soccer match."
"A team wins and the other loses," he told reporters.
But regime concerns over a third day of demonstrations mounted as truckloads of riot police crisscrossed the city and motorcycle units swarmed the streets.
Most of the protests took place at university dormitories, a few residential areas and the expensive northern suburbs that climb the foothills of the Alborz mountain chain. There were fewer demonstrators and the violence was more localized Sunday than Saturday. In Vali Asr Avenue, the longest street in Tehran and the spine of the protests, motorists practiced passive resistance, driving up the crowded avenue shortly after sunset to a cacophony of horns.
Members of the Basij, a paramilitary group, stood in the middle of the streets with sticks in their hands threatening motorists honking their horns.
"The Internet is not working. There is no SMS and Facebook is filtered, but news of protests is being communicated by spray painting on walls," said Laleh, an arts student at Amir Kabir University who would give only her first name, as she leaned out her window flashing victory signs. "It's like a revolution."
Mr. Mousavi, the defeated candidate, said on his Web site that he had formally asked the Guardian Council, a legislative body, to cancel the election result.
He also urged that demonstrations on behalf of his candidacy continue.
"I urge you, Iranian nation, to continue your nationwide protests in a peaceful and legal way," he said.
The protests, which erupted Friday, have been the worst since 1999 when police cracked down on students protesting the closure of a reform newspaper and several students died.
"It's just like the events of [July 9, 1999]," said a political analyst who asked not to be named to avoid retribution from the government. "In a week, the supreme leader will give a talk at Enghelab [Revolution] Square, declare the coup a success and it'll be official. The complication now is that it's split our society into two parts."
The polarization was palpable on the streets as Ahmadinejad supporters driving cars decked out with Iranian flags traded insults with Mousavi followers or pointedly avoided staring at one another as their cars waited at traffic lights.
"Some people used Mousavi as a front in order to follow their own ambitions," said Fatemeh Mohammadi, a graduate of economics who attended Mr. Ahmadinejad's victory rally. "And now these people will be judged by Mr. Khamenei. Their fate will be decided."
As night fell, more protesters took to the streets, dragging burning barricades and setting fire to trash bins. Crowds stood at major intersections.
"America and Israel are behind these movements," said Maryam Abdollahi, an Ahmadinejad supporter resting with her university graduate friends in an alley after Mr. Ahmadinejad's televised news conference. "They've been waiting for an excuse to enter our country and have been frustrated [by Mr. Ahmadinejad's re-election]."
Some protesters took to their roofs and began chanting, "Allah o Akbar" [God is great], a phenomenon reminiscent of the 1979 revolution that overturned the monarchy.
Demonstrations were also reported in the cities of Tabriz, Rasht, Sari and Mashhad.
State media quoted a police deputy commander, Ahmad Reza Radan, as saying that police had detained nearly 100 people who reportedly instigated protests and 10 prominent plotters, among them Mohammed Reza Khatami, the younger brother of former President Mohammed Khatami.
As uncertainty spread and state-run television continued with normal programming, rumors swirled through the capital that dozens of other reformist politicians had been arrested including almost the entire central council of the Islamic Reformist Participation Front, the main reformist political party.
There were also reports that two reformist clerics, Ayatollahs Sanei and Mousavi Ardebili, had been placed under house arrest in the theocratic center of Qom. There was an uncharacteristic silence from Mehdi Karroubi, another presidential candidate who had vociferously protested what he claimed was rigging in a 2005 vote.
The state-run media accused foreign journalists of contributing to the protests, and there were reports that the office of Al Arabiya television, an Arabic language station based in Dubai, United Arab Emirates, had been closed for a week.
The BBC reported heavy jamming of its Farsi language news service, and rumors swirled that foreign journalists were about to be expelled.
"In our questioning we're after finding links between the plotters and the foreign media," the state-run Fars news agency quoted Mr. Radan, the deputy police chief, as saying.
Text messaging remained blocked, leaving Web sites and land lines to convoy protesters' messages.
Mehdi Jedinia contributed to this report from Washington.