Ahmad Reza Radan, Tehran's deputy police chief, told the official IRNA news agency that at least 60 people had been detained over the rioting and more arrests would be made soon.
Tthere were reports of renewed clashes in Tehran on Sunday as police confronted about 200 stone-throwing protesters.
"Forces used tear gas in some areas to stop the unrest. The situation is under control," Radan said.
'Major crackdown'
Al Jazeera's Alireza Ronaghi, reporting from Tehran, said that IRNA was reporting that a committee led by two senior supporters of Mousavi was organising the riots against the election result.
"Whether this is really an honest outburst of anger against the outcome of the election we don't know yet, but what we see is a major crackdown on reformists and their leaders," he said.
Mohammad Reza Khatami, the brother of Mohammad Khatami, a former president, was among the members of the Islamic Iran Participation Front, which won more than 100 seats in the 290-member parliament in 2000, who were arrested.
"They were taken from their homes last night," Mohammad Ali Abtahi, a former Iranian vice-president, said.
Ahmadinejad was declared the winner of Friday's polls by a wide margin, with figures from the interior ministry showing he had taken 62.63 per cent of the vote, while Mousavi, his main challenger, garnered only 33.75 per cent.
Ahmadinejad defended the conduct of the election at a news conference at the presidential office in Tehran on Sunday.
He also questioned Mousavi's claims that the vote was "rigged".
"It is not clear how they are questioning the participation of 40 million people in these elections," he said.
"I have still not been shown any documents by anybody, they are just saying the outcome in unexpected.
"It is like football, everybody expects their team to win."
Reformists detained
Sadegh Zibakalam, head of the Iranian studies department at Tehran university, told Al Jazeera that the demonstrations were largely "spontaneous" responses to the election result.
"No one is giving them commands, no one is ordering them, no one is leading them," he said.
"Nevertheless, the government has started a crackdown on the leading reformist figures ... The government reaction is too harsh, but it is understandable."
Commenting on the dispute, Mehran Kamrava, director of the centre for international and regional studies at Georgetown University's campus in Qatar, told Al Jazeera that Iranian elections are "notoriously unpredictable".
"The Western media has been talking to people in north Tehran, who tend to vote overwhelmingly against Ahmadinejad," he told Al Jazeera.
"But let's not forget that many of the urban Iranians have priorities and proclivities that are not necessarily reflected in other areas of the main cities, and those people could easily have voted for Ahmadinejad.