Iran has cracked down on women violating its compulsory headscarf decree, arresting at least 29 individuals, according to Iranian media, and drawing criticism from activists and rights groups.

Tasnim news agency reported on Friday that 29 women had been arrested, and quoted Tehran police as saying that the detainees were arrested for "disturbing public security".

It was unclear where else arrests had been made, as protests have spread from the capital to other areas, including the ancient cities of Esfahan and Shiraz.

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The mandatory headscarf, or hijab, has been in place in Iran since 1979, after the Iranian revolution and the installation of Ayatollah Khomeini as Supreme Leader.

Over the years, hundreds of thousands of Iranian women have protested against the law.

In recent weeks, following a new wave of protests in the country, women renewed their opposition to the law, taking off their hijabs in public and waving them on wooden sticks like flags.

Holly Dagres, an Iranian-American analyst, said Iranian authorities are "very much aware" that more than half of the population is against wearing the hijab.

"It's evident by the fact that the morality police are on constant patrol of the streets of major cities like Tehran," Dagres told Al Jazeera.

"Authorities know that if they don't crack down, Iranian women will continue to test the boundaries of what they can and cannot wear."

Dagres said that more arrests were likely to spur solidarity with the campaign.

Two friends, one against the hijab, the other for, have sent their pix to @RadioFarda_ from Yasouj in southwestern Iran to express support for women protesting compulsory hijab #دختران_خیابان_انقلاب pic.twitter.com/FlRwYNyAJS — Golnaz Esfandiari (@GEsfandiari) February 1, 2018

On social media on Friday, Masih Alinejad, an exiled Iranian activist who started the campaign against the compulsory hijab, singled out Iranian Foreign Minister Javad Zarif for "remaining silent" on the arrests.

'Small acts of resistance'

While hijab protests are not new, Dagres told Al Jazeera that recent rallies were inspired by a lone female demonstrator, who stood on a busy pavement in central Tehran waving her white headscarf on a wooden stick.

The image of the woman spread on social media.

Her case attracted more attention when she was reportedly detained by police in late January. She was later released on bail, according to the Iran Human Rights group.

In another viral video, an elderly woman was shown struggling to walk in a snow-covered park, then clambering up an idle fountain to wave her white scarf in the air.

Iran Human Rights said that another woman, who also protested, had been transferred to a prison south of Tehran, and that she is being held on bail set at $135,000.

On Wednesday, Mohammad Jafar Montazeri, Iran's chief prosecutor, had denounced the protests as "childish", suggesting that individuals from outside Iran are inciting the movement.

Under Iran's law, a woman who does not wear a hijab in public could face jail time or fines.

While only a few of Iran's more than 80 million people have joined in the headscarf protest, social media applications such as Telegram and Instagram have helped spread the news quickly.

Some hijab-supporting Iranian women back the protests.

One Iranian woman, wearing the traditional black outer garment, or chador, was seen standing on top of a utility box in the corner of a busy Iranian street expressing solidarity.

Omid Memarian, a US-based Iranian journalist who was once imprisoned in Iran, wrote on social media that the fight against forced hijab "is not about whether the hijab is good or bad. It's about choice and equality. It's about dignity".

So far, the protest still lacks momentum for Iran's authorities to consider amending the hijab law, said Dagres.

"At the same time, movements always start as small acts of resistance. If these protests gain traction, we might see a shift," she said.

She added that when it comes to women's rights in Iran, amendments to the marriage and divorce laws need greater attention.