Greek authorities have launched an investigation into allegations that riot police used a female protester as a human shield during angry demonstrations over a visit to Athens by the German chancellor, Angela Merkel, this week.

Witnesses told the Guardian the young woman, who has yet to be identified, was frogmarched in handcuffs ahead of riot police as protesters threw stones at officers.

News of the investigation came as magistrates launched a separate inquiry into a Guardian report that anti-fascist protesters, arrested after clashing with extremists from the neo-nazi Golden Dawn party, were subjected to torture by officers at the Attica General Police Directorate. Human Rights Watch said accountability for police abuse was urgently needed.

The group said: "The scenes described by the victims to reporters are deeply shocking. No one should be treated that way by police. Greece needs to conduct a thorough and impartial investigation of their allegations."

The inquiry into the human shield allegations was opened after photographs of the incident began to circulate on the internet, triggering condemnation of the tactics law enforcement officials stand increasingly accused of employing in Greece. "It is being investigated," said Lieutenant Colonel Christos Manouras, Greece's police spokesman. "We want to find out what these pictures hide."

In the pictures, a young woman, her faced daubed in white anti-teargas solution, with a pink rucksack on her back, is seen being escorted by riot police before being placed in front of the unit when it encounters stone-throwing protesters. Witnesses said the woman appeared to be disoriented and terrified as she was marched through Athens in handcuffs.

Foula Pharmacides, a shipping company employee, had participated in the demonstration outside parliament, but fled down a side street off Syntagma Square when police fired teargas to disperse the crowds. She described terror on the woman's face. "As the squad moved down Xenofontos Street with the girl, the protesters appeared," she said. "Then when the protesters started throwing things, the cop holding the girl takes her from the front of the unit to the back to face them and he starts moving her like a shield from left to right.

"The girl was falling down and he was picking her up. She was crying and clearly terrified. I couldn't believe it. You only ever see this sort of thing in the movies. Everyone started screaming 'Shame on you! Shame on you!' I remember there were two women next to me and they were crying, too, and screaming for the police to stop."

Sokratis Michalopoulos, another witness, said the episode ended when the "booing got so loud" and the riot squad decided to move on. "I don't think I will ever forget her face," said the 36-year-old television technician. "It was as if she were an object not a human being and I think she was in shock. She was definitely being used as a shield. Thank God photographers were there and we now have cameras on phones otherwise people would think we were mad. No one would believe us."

Veteran photographer Spyros Tsakiris, who also witnessed the incident, said the riot policeman who had been holding the woman had written "killer" in English on one of his bag straps.

As Greece descends into further chaos amid mounting social and political tensions, accusations of police brutality are growing. In recent months, reports have multiplied of codes of conduct being flouted by law enforcement officers.

The heavy-handedness has been attributed to links between the police force and the far-right Golden Dawn, whose popularity has surged on the back of soaring crime and anti-immigrant hysteria.

"The police are angry. They are overstretched and underpaid and becoming increasingly anti-government and radicalised in this case to the right," said Panos Garganas, a prominent leftist activist.

"Since democracy was restored in 1974 [with the collapse of military rule] I have attended hundreds of demonstrations and have seen police sit on people and kneel on people, but never someone being used as a human shield," he said.