The police bus screeches to a halt at a Tehran square packed with traffic and officers leap out to begin spot checks on passing pedestrians and cars.
It's police work apparently like any other place in the world, but here in the Iranian capital their targets are women deemed to have infringed the Islamic republic's strict dress rules.
"For God's sake no pictures!" yells a mother whose daughter has just been stopped by the male officers for her Islamic headscarf (hijab) being pushed too far back and revealing an excessive amount of hair.
The dusk patrol in Tehran's western quarter of Shahrak-e Gharb is part of a nationwide crackdown aimed at "guiding" women to adhere to the Islamic dress code, which since the 1979 revolution requires women in Iran to cover their heads and bodies.
The authorities insist that the drive is more aimed at encouragement and Islamic guidance than coercion, with arrest a last resort if women show a reluctance to change their ways.
"When we stop a vehicle, we politely tell them to correct their hijab. If our advice is carried out, then we leave it at that," police Corporal Habib Mohammad told an agency reporter who was taken on the patrol.
"If not and the female passenger or driver shouts back, then we will ask her for her car's document, and we will stop her car and take her case to the police station."
The crackdown is a regular event in late spring in Tehran ahead of the hot summer but appears to have been flagged with more prominence in the media and is being pursued with more vigor by authorities this year, as hemlines get higher and headscarves skimpier.
The boldness of some women in Tehran in showing fashionably styled hair peeking out from beneath their headscarves, wearing trousers that reveal naked ankles and figure hugging mantos (coats) has infuriated conservatives.
Hardline sections of the press and conservative MPs have vehemently backed the police decision to enforce a "combatting and guidance drive" against the wishes of those who preferred a more softly-softly approach.
"The current situation is shameful for an Islamic government. A man who sees these models on the streets will pay no attention to his wife at home, destroying the foundation of the family," said Mohammad Taghi Rahbar, a member of the culture committee of the Iranian parliament.
When the conservative Mahmoud Ahmadinejad was elected president in June 2005 there were expectations that the authorities would clamp down firmly on women's dress in public. But fashions on the streets show nothing has changed.
A media advisor to Ahmadinejad issued a statement thanking Iran's police chief Esmaeel Ahmadi Moghadam for "promoting virtues and combatting vice" with the new crackdown.
"I see that your force has entered combat with this cultural invasion and, in its dirtier form, 'cultural NATO' through scientific means and I sincerely thank you and your colleagues," said Mehdi Kalhor.
However the exchanges between police and public on the street showed that the approach has not won universal popularity.
"This is a not good plan, the way they carry it out," said a young woman who would only identify herself as a clerk.
"Why should they bother themselves with what people wear? Their presence increases one's level of stress. Our parents are okay with what we wear, why should they care?" she said.
"If they want to stop vice, then why are these clothes imported? If they really want to deal with the problem then they should prevent the merchants from importing these mantos, pants and scarves," said a shopkeeper.
A bearded man who identified himself as Mr Mohammadian, a civil servant approached reporters to complain: "They arrested my daughter last night on Jordan street. She was sitting in her friend's car.
"They were stopped for a few strands of hair, her friend's car was impounded. Students should be treated with care. If they are bothered they will leave Iran, we need to keep these people at home," he told a police officer.
"I am a war veteran, I paid my dues for the revolution, please, I plead with you, go and check, there was a car impounded last night on Jordan Street with two girls in it. This is not the way to treat our youth."
The head of Tehran's police information centre, Colonel Mehdi Ahmadi replied: "Certainly, in carrying out a large scale operation, there can be some degree of error. We will look into it."