Despite a heavy police presence, approximately 4,000 people massed in Tehran's Azadi Square and more were streaming in, The Wall Street Journal reported Monday.
In the Yemeni capital of Sanaa, pro- and anti-government protesters clashed.
In Algeria, government officials announced plans to lift a 20-year state of emergency.
In Tunisia, the EU Foreign Policy chief met with interim government leaders.
Several thousand protesters reportedly gathered in Tehran's Imam Hussein Square where they sat on the ground and chanted when police attempted to remove them, the newspaper said.
Protesters took to the streets despite arrests of opposition leaders and activists and public announcements calling on Iranians to refrain from participating in the protests, the newspaper said.
Thousands chanted "death to the dictator" and "down with the Taliban, in Cairo and Tehran," The Washington Post reported.
Before the protests, opposition leaders Mir Hossein Mousavi and Mehdi Karroubi were placed under house arrest and lines of communications, including mobile phones, were cut off, the Journal said, quoting opposition Web sites.
Police blocked streets near the leaders' homes and reportedly prevented Mousavi's wife from leaving, the Journal reported.
"These people know very well their request is not legitimate and they know they will never get a permit to stage riots," Mehdi Alikhani deputy chairman of the political bureau at Iran's Interior Ministry, told the semi-official Fars news agency.
Bloggers complained of censorship and access to hundreds of news Web sites was blocked..
The U.S. State Department launched a Farsi-language Twitter feed Sunday in a bid to communicate with young Iranian Internet users.
"US State Dept recognizes historic role of social media among Iranians. We want to join in your conversations," the department said in its first message over the microblogging service.
"The regime's concern is that mass demonstrations in support of the Egyptian and Tunisian uprisings will turn into demonstrations against the Iranian regime itself," the Middle East Media Research Institute said Monday.
This fear was reflected in a statement made by an Iranian commander, Mohammed Reza Naqdi, who said western intelligence agencies were looking for a disabled person to set himself on fire in Tehran to spark"a popular uprising, the Institute said.
Days after protesters took to the streets in Algeria demanding government reform, authorities Monday announced a 20-year state of emergency would be lifted in the "coming days," CNN reported.
"Soon we will discuss the past, but I say that lifting the state of emergency will occur in the coming days," Algerian Foreign Minister Mourad Medelci told French radio Europe 1.
European Union foreign policy chief Catherine Ashton visited Tunis Monday and met with Prime Minister Mohamed Ghannouchi and other government officials, Radio Free Europe Radio Liberty reported. An interim government was set up in the country after an uprising forced the former president of 23 years into exile last month, the radio said.
In Yemen's capital, pro- and anti-government protesters clashed for the third day in a row, throwing stones at each other, and in some cases batons and knives were wielded.
CNN reported 200 anti-government protesters rallied outside Sanaa University, demanding regime changes. They were confronted by a group of government supporters carrying pictures of President Ali Abdullah Saleh.