Yemeni female security forces descended on a small women's rally in the southern port city of Aden on Wednesday, beating the women and dispersing the gathering, activists said.
The policewomen stripped one protester of her niqab, the all-covering face veil worn by conservative Muslim women, then punched her repeatedly in the face, according to the activists who organized and joined the demonstration.
Two women were briefly detained and interrogated, they added, speaking on condition of anonymity for fear of retribution.
The rally took place near troop headquarters of the United Arab Emirates, which is part of a Saudi-led coalition backing Yemen's government in the country's civil war with northern Shiite rebels, in control of the capital, Sanaa.
The protesters - mothers and sisters of scores of detainees held without charges at undisclosed locations - chanted: "Oh coalition, where are our sons?"
One of the protesters, a 34-year-old teacher, said she joined the rally because her 24-year-old brother, a university student, was snatched off the streets of Aden last August along with their uncle. While the family learnt about the whereabouts of the uncle in the meantime, the brother's whereabouts remain unknown, she said.
"Our sons and brothers vanished," she told The Associated Press over the phone. "For a year, we know nothing about them."
Like many others, she said, her brother was a fighter with southern factions who were backed by the coalition in 2015 in the war against the Houthi rebels who tried to seize Aden.
The teacher said that her uncle is held in Aden's central prison, where the coalition has taken control of a section where it keeps terror suspects.
Last month, an investigation by the AP found that hundreds of southern Yemeni men vanished into a network of secret prisons run by the UAE and loyal forces, including the so-called Security Belt in Aden. Arrests of detainees are based on suspicions the detainees belong to al-Qaida branch in Yemen.
The UAE has denied running such prisons. Yemeni families and former inmates have told the AP that torture and abuses are rampant in these prisons.
Last year, families staged near-weekly protests against the men's disappearances but some of these rallies were disbanded by force while organizers were intimidated and faced threats of detentions.
"We went to the presidency, to the coalition, and to the security chief but no one is talking to us," the teacher-protester said. "The militias are running the country."
Yemen's stalemated civil war, now in its third year, has killed more than 10,000 civilians, displaced over three million people and pushed the Arab world's poorest country to the brink of famine.
The country's economy has been bombed into ruins, with a new misery being added: the world's biggest current outbreak of cholera, with more than 300,000 cases. More than 1,700 people, over a quarter of them children, have died of cholera since the end of April.