Given foreign media restrictions in Iran, it remained difficult to confirm details of the weekend's violence. The BBC estimated tens of thousands of protesters were in the streets on Saturday.
As one crowd made its way toward Tehran's Revolution Square, reports said, it found the square closed off by security forces, including Revolutionary Guard forces and plainclothes members of the Basij militia. Those forces, bystanders said, assaulted protesters and chased them through the streets.
"This is a very unfair fight. They are using chains, hoses, clubs, they are carrying shotguns ... and they are brutal," one eyewitness in Tehran told ABC News in the midst of the clashes.
An amateur video posted to YouTube apparently showed the shooting death of one protester; sources said she was shot in Amir Abad, a neighborhood in Tehran.
Witnesses speaking with ABC News by e-mail described the clashes as a "street war."
"War is going on," one witness said, another describing "tear gas and burning trash cans everywhere."
"I feel like I'm in a police state for the first time. ... As of late afternoon, Basij are everywhere," said another protester, as the force was seen violently breaking up protests.
The International Campaign for Human Rights in Iran said forces pursued the wounded and arrested them in Tehran hospitals.
Justin McMahan, an American traveling through Iran with his wife when the protests began last week, was staying at a hotel near Engelab Square and saw the violence.
"We saw people getting indiscriminately clubbed by the Basij, beaten with batons, though they weren't wearing any green," he told ABC News. "There was unbelievable tension in the city, it was a place in turmoil. You could feel the society is deeply divided."
"There was talk of around 3,000 people were in Engelab Sq., said McMahan's wife Andrea. "We saw hundreds of military and Basijis in plainclothes.
"There were women screaming, gunshots fired," she added. "Police were brutally beating people, it was totally chaos. And it was hard to get any info. State TV stations were playing sports games."
By night protests had taken on a different form, channeled into what have become nightly neighborhood chants on city rooftops.
"The shouts of Allaho Akbar in holy city of Mashhad are 'explosive,'" said one source, "loudest it has ever been."
Others describe the chanting and shaking as "surreal."
The rooftop "Allaho Akbars," meaning "God is great," were widely as a means of protest during the 1979 Islamic Revolution. Sources said the practice has again become popular, given its relative safety -- largely out of reach of riot police in the streets.
The protesters directly defied Iran's highest authority, Supreme Leader Ayatollah Ali Khamenei, who in a Friday prayer speech called on them to stop demonstrating and accept President Mahmoud Ahmedinejad's reelection.
One week earlier, Ahmedinejad was declared the winner in a presidential ballot that gave him a commanding, some said implausible landslide win. Khamenei's speech warned that opposition leaders would be responsible for the consequences of any demonstrations, which some analysts saw as a veiled threat of violence.