An Iranian woman who made headlines around the world when she was filmed waving a white headscarf in the street, without wearing a hijab, has been reported missing.

The 31-year-old, who has not been named, had her head uncovered as she appeared to be protesting against the country's mandatory Islamic dress code.

She was arrested shortly after the display and hasn't been seen since, according to prominent human rights lawyer Nasrin Sotoudeh.

"After being arrested, she was released shortly and then re-arrested," Ms Sotoudeh wrote on her Facebook page.

She added that the woman had been transferred to a police station and then a prosecutors office.

In a separate post, she urged authorities to reveal where she was being held.

"Where is our sister?" she wrote, adding that she had not been in contact with the woman's family, but understood she had a 19-month-old son.

A video of the woman's gesture was posted on Twitter by Armin Navabi, a former Muslim from Iran and author of Why There Is No God.

“This woman in Iran took off her Hijab to protest the mandatory Islamic dress code imposed on Iranian women,” he tweeted, using the hashtag “#IStandWithHer”.

His tweet has been retweeted over 15,000 times and liked more than 32,000 times and many praised her for her action.

“This woman is brave,” one wrote, while others called her a “real hero” and said “her bravery is to be admired.”

Iran protests – in pictures Show all 11 1 /11 Iran protests – in pictures Iran protests – in pictures University students at an anti-government protest inside Tehran University, 30 December 2017 AP Iran protests – in pictures A university student at a protest inside Tehran University while a smoke grenade is thrown by anti-riot Iranian police, 30 December 2017 AP Iran protests – in pictures University students at a protest inside Tehran University, 30 December 2017 AP Iran protests – in pictures University students run away from the police during an anti-government protest inside Tehran University, 30 December 2017 AP Iran protests – in pictures An image grab taken from a handout video released by Iran's Mehr News agency reportedly shows a group of men pulling at a fence in a street in Tehran, 30 December 2017 AFP/Getty Iran protests – in pictures Demonstrators gather to protest in Tehran, 30 December 2017 AP Iran protests – in pictures Iranians chant slogans as they march in support of the government near the Imam Khomeini grand mosque in Tehran, 30 December 2017 AFP/Getty Iran protests – in pictures Iranians chant slogans as they march in support of the government near the Imam Khomeini grand mosque in Tehran, December 30 2017 Iran protests – in pictures Iranian clerics take part during a state-organized rally against anti-government protests in the country, in the holy city of Qom, south west Iran, 3 January 2018 EPA Iran protests – in pictures In this photo provided by the Iranian Students' News Agency, a clergyman takes a picture of a pro-government demonstration in the southwestern city of Ahvaz, Iran, 3 January 2018 ISNA via AP Iran protests – in pictures Pro-government demonstrators gather at the Massoumeh shrine in Iran's holy city of Qom, some 130 kilometres south of Tehran, 3 January 2018 AFP/Getty

It is thought she was protesting as part of the "White Wednesdays" online campaign against compulsory headscarves. Other women have filmed themselves in confrontations with people on the street about the issue.

Women in Iran have been forced to cover their hair since the 1979 revolution.

But last week, police in the country's capital Tehran announced they would no longer arrest women for failing to observe the Islamic dress code.

However, the semi-official Tasnim news agency reported that violators would instead be made to attend classes given by police officers. Repeat offenders could also still be subject to legal action, it added.

Younger and more liberal-minded women have long pushed the boundaries of the country's official dress code by wearing loose headscarves that don't fully cover their hair and painting their nails.