"Police fired tear gas as a cat-and-mouse game (between police and demonstrators) began in Vali Asr Square" of Tehran, the Rahesabz.net said citing witnesses.
Crowds of demonstrators were also gathering in other Tehran squares and some streets, shouting "Allahu Akar" (God is Greatest), the Sahamnews.org website of opposition leader Mehdi Karroubi, said.
It also reported that gatherings were staged in the central city of Isfahan and in the southern city of Shiraz, while Fars news agency reported that situation was calm in Iran`s second largest city of Mashhad in the northeast.
Fars said that "calm" prevailed in Tehran in the face of the presence of security forces in "full strength."
"The police was in control of the situation and there was peace in the city with no reports of any incidents," Fars said.
Foreign media have been banned from covering any opposition gatherings.
Tehran traffic police chief Hossein Rahimi confirmed to the state news agency IRNA that security forces were deployed in the capital.
"Traffic was heavy on some roads due to the presence of police and special forces," Rahimi said, but added that no part of Tehran was blocked and no traffic restrictions were imposed.
"The traffic was flowing," he said.
Witnesses and opposition website Kaleme.com reported that men on motorbikes were riding in parts of Tehran "in a show of strength... to intimidate people and prevent them from holding large gatherings."
Rahesabz.net reported that mobile networks in central Tehran were cut, while the Internet speed was "noticeably slow."
The Sunday demonstrations were called by a group owing allegiance to opposition leaders Karroubi and Mir Hossein Mousavi.
The group, Council for Coordination of Green Movement Hope, on Thursday posted on opposition websites the call for Sunday gatherings to mark the seventh day after the killing of two people in protests in Tehran on Monday.
Those protests, which saw demonstrators clash with security forces, were the first since February last year when the authorities marked the 31st anniversary of the Islamic revolution.
Nine security personnel were injured in Monday clashes, officials and websites said.
Anti-government protests erupted in Iran after a June 2009 presidential election, in which Mousavi and Karroubi, both candidates, charged that the incumbent Mahmoud Ahmadinejad was fraudulently re-elected. (*)