Alireza Ronaghi, Al Jazeera's correspondent in Tehran, said that the events "mean that everything is becoming very serious" as the typical situation of secret security services undertaking crackdowns has changed.
"We have so far seen that Iranian police have been staying on the sidelines ... but right now it is the police who are intervening," he said.
"So now it's police and the people."
The comment came as Mir Hossein Mousavi and Mehdi Karoubi declined to meet the Guardian Council, Iran's highest legislative body, concerning 646 complaints of voting irregularities in the June 12 poll.
The council had called for a meeting to discuss the vote, which both Mousavi and Karoubi have said should be annulled.
State television quoted a council spokesman as saying that the Guardian Council had expressed its readiness to "randomly" recount up to 10 per cent of the ballots.
The contested result gave President Mahmoud Ahmadinejad a tally of about 63 per cent, to Mousavi's 34 per cent. The council has so far promised only to recount some ballots if irregularities are found.
Ronaghi said that it was likely that the meeting would bring only minor changes to the poll outcome.
"I think the Guardian Council intends to review all of the complaints," he said.
"In some of those cases, there are some obvious irregularities and I am sure that they will be addressed. I'm sure some of the ballot boxes will be annulled and they will be deducted from the results.
"But ... I find it very unlikely that there will be major change in the final result."
The meeting comes a day after Ayatollah Ali Khamenei, Iran's supreme leader, backed Ahmadinejad's victory and cautioned that mass protests - which continued daily from the release of the poll results until Thursday - must cease.
Khamenei said in a sermon during Friday prayers at Tehran University that if the supporters of defeated candidates fail to halt the protests "they will be responsible for its consequences, and consequences of any chaos".
But an indication that resistance to the results wouldl continue came before dawn in Tehran on Saturdayas opposition supporters called "Death to the dictator" and "God is the greatest" from the rooftops.
Television crews are prevented from filming rallies and many non-Iranian journalists have been prohibited from working in the country.