More than 100 reformists, including the brother of former Iranian president Mohammad Khatami, were arrested Saturday night, leading reformist Mohammad Ali Abtahi told Reuters.
"They were taken from their homes last night," former vice president Abtahi said.
Authorities released the former president's brother, Mohammad Reza Khatami, on Sunday, his wife said. She said at least two other top leaders of Iran's largest reformist party, including the party's secretary-general, were also released early Sunday, but that others remained in custody.
Additional arrests were expected.
Iranian officials, however, disputed the claims and said the reformists were merely summoned and "warned not to increase tension."
Tehran deputy prosecutor, Mahmoud Slarkia, told the semi-official ISNA news agency that less than 10 people were arrested on the charge of "disturbing public opinion" through their "false reports" on Web sites after the election.
Tehran was mostly calm Sunday after election fraud claims triggered violent street clashes, but the government maintained fairly tight control of information flow and new details emerged of arrests of high-profile reformists.
The efforts seemed aimed at avoiding a repeat of the chaos that lasted past midnight Saturday. Opponents of President Mahmoud Ahmadinejad set buses and cars ablaze in the capital and threw rocks at police to protest what they viewed as his illegitimate victory.
Ahmadinejad reportedly was due to hold a news conference Sunday before attending what is expected to be a huge victory rally.
Iran restored cell phone service that had been down in the capital since Saturday. But Iranians could not send text messages from their phones, and the government increased its Internet filtering in an apparent bit to undercut liberal voices.
Web sites linked to reformists' new hero Mir Hossein Mousavi, who declared himself the true winner of Friday's presidential race and urged backers to resist the government, were down. Social networking sites including Facebook and Twitter were also not working.
The restrictions were likely intended to prevent Mousavi's supporters from organizing large-scale protests. But several small groups took to the streets, according to witnesses. About 300 Mousavi supporters gathered outside Sharif University, chanting "Where are our votes?"
About a dozen riot police descended upon a crowd of some 50 Mousavi supporters standing outside his campaign quarters, beating them with batons and causing them to disperse.
Reports that Mousavi was under house arrest could not be confirmed, but the 67-year-old former prime minister has not been seen in public since he gave a late night press conference Friday where he accused the government of voter fraud. On Saturday, Mousavi released a Web message saying he would not "surrender to this manipulation."
Iran's deputy police chief Ahmad Reza Radan said some of Saturday's protesters were detained and police used tear gas to stop the demonstrations. He said the situation was under control and accused the foreign media of exaggerating the protests to show unrest in Tehran.
"Police will not allow protesters to disturb the peace and calmness of the people under the influence of foreign media," Radan said on state television, which showed footage of the protests for the first time Sunday.
Ahmadinejad also accused the foreign media of producing coverage that harmed the Iranian people in an address to the country broadcast on state TV late Saturday, saying "a large number of foreign media ... organized a full-fledged fight against our people."
He did not mention the unrest, saying only "a new era has begun in the history of the Iranian nation." Ahmadinejad is scheduled to hold a massive rally with supporters Sunday afternoon in central Tehran.
Slarkia, the deputy prosecutor, confirmed that Iran was blocking five pro-Mousavi Web sites because of election violations. He did not elaborate.
Mousavi's newspaper, Kalemeh Sabz, or the Green Word, did not appear on newsstands Sunday. An editor, who spoke on condition of anonymity because of the sensitivity of the situation, said the paper never left the printing house because authorities were upset with Mousavi's statements.
The paper's Web site reported that more than 10 million votes in Friday's election were missing national identification numbers similar to U.S. Social Security numbers, which make the votes "untraceable." It did not say how it knew that information.
Thousands of Mousavi supporters took his call to the streets on Saturday, setting trash bins and tires ablaze. Police fought back with clubs, including mobile squads on motorcycles swinging truncheons.
Iran's supreme leader, Ayatollah Ali Khamenei, closed the door for possible compromise. He could have used his near-limitless powers to intervene in the election dispute. But, in a message on state TV on Saturday, he urged the nation to unite behind Ahmadinejad, calling the result a "divine assessment."
The U.S. has refused to accept Ahmadinejad's claim of a landslide re-election victory said it was looking into allegations of election fraud. There are no independent election monitors in Iran.
U.S. Secretary of State Hillary Rodham Clinton said Saturday she hoped the outcome reflects the "genuine will and desire" of Iranian voters.
The European Union also said it was "concerned about alleged irregularities" during Friday's vote.
Past Iranian elections were considered generally fair. In 2005, when Ahmadinejad was first elected, the losing candidates claimed irregularities at the polls, but the charges were never investigated.
Mousavi called on his backers to avoid violence, but he is still talking tough about pressing his claims of election fraud. He charges the polls closed early but has not fully outlined all of his fraud allegations.
There also have been no hints of any new policy shifts on key international issues such as Iran's standoff over its nuclear program and the offer by President Barack Obama to open dialogue after a nearly 30-year diplomatic estrangement. All high-level decisions are controlled by the ruling theocracy.
The Associated Press contributed to this report.