By day, the city of over eight million people, dramatically framed by a mountain range, looks peaceful, with children heading off to school and shops welcoming customers.

At night, over the past 10 days, the city has changed its face as protesters have taken to the streets, facing off and clashing with security services.

A wave of unrest has rocked Iran since the death of 22-year-old Kurdish woman Mahsa Amini on September 16 following her arrest by the morality police for reportedly not observing Iran's strict dress code.

The protest movement, which has spread to other cities in the country, has led to the deaths of dozens of people, mostly protesters but also members of the security forces.

But after the overnight chaos, Tehran in the mornings looks almost like any other big city.

"In the morning, my wife takes the children to school and I open the shop," said Mahmoud, 60, who sells mobile phone accessories.

"In the evening, when the protests start ... business goes down and we have to close the shop."

Mahmoud's shop is located on Valiasr Square in central Tehran, where police officers with helmets and armed with truncheons have taken up positions.

- 'My heart ached' -

Tehran's police force has been so busy that the Iranian judiciary chief, Gholamhossein Mohseni Ejei, paid them a morale-boosting visit on Sunday.

The officers have been deployed "24 hours a day", said Ejei, adding that "they did not sleep last night and the nights before ... and they must be thanked."

During the daytime, the iconic Valiasr Square is full of people, traffic is heavy, and customers abound in the shops.

"The Tehran of the morning is a normal city, but at night during the protests, as you have seen, it is the scene of arson attacks," said Marzieh, an 18-year-old journalism student.

Marzieh, wearing a dark chador, was critical of the protests: "We can no longer call them demonstrations ... They have morphed into a riot."

The chants of demonstrators have echoed through more distant neighbourhoods too, away from the busy centre.

Ali, who lives in Tehran's northwest, said that for several hours every evening he can hear people shout "Woman, Life, Freedom" from windows and rooftops.

The night-time protests are in part dictated by the rhythm of daily life, said Aaraam, 30, a lawyer wearing a fashionable grey headscarf and sunglasses.

"People are at work in the morning and have no time, so they come out protesting in the evenings," she said.

"People's feelings have really been hurt ... As an Iranian, my heart really ached when I saw how upset my country's people are, and they are making rightful demands."