When police arrested Tayib Mohammed at the southern border of Saudi Arabia, they seized all his worldly possessions and set them on fire.

The 45-year-old undocumented Ethiopian migrant was trying to cross from Yemen after a five-day trek through the bush. “They told me to undress,” he recalled several weeks later in the Ethiopian capital, Addis Ababa, wearing sandals and pyjamas. “They took away everything I had – phone, clothes, money. They burned them in front of me.”

Tayib is one of roughly 10,000 Ethiopians deported every month from Saudi Arabia since 2017, when authorities there stepped up a hardline campaign to remove undocumented immigrants. Roughly 300,000 have returned since March of that year, according to the latest International Organization for Migration (IOM) figures, with special flights loaded with deportees arriving at Addis Ababa airport each week.

The number of Ethiopians living in Saudi Arabia is unknown, but before the start of the campaign there were thought to be about half a million, often working low-skilled and poorly paid jobs in construction and domestic service. Most reach the kingdom via the perilous Red Sea crossing from Djibouti to wartorn Yemen, which gets little international attention but receives a much higher number of undocumented migrants than the Mediterranean crossing to Europe. Most are escaping poverty and unemployment back home.

Hundreds of thousands of Ethiopians were deported in a chaotic previous crackdown conducted between 2013 and 2014. Though the numbers are higher this time, the situation has gone largely unnoticed.

“They’ve systematised the deportation machine now,” said Adam Coogle of Human Rights Watch (HRW).

Police detain illegal African immigrants near Thamama, in the Saudi capital Riyadh. Photograph: Reuters

Deportees like Tayib return empty-handed and report serious abuses by Saudi police and prison guards. A few have returned in such poor health that they died shortly after arriving at the airport in Addis Ababa. Some had untreated bullet wounds.

Saudi Arabia is one of few countries not to have ratified the main international treaties relevant to immigration detention. According to the Geneva-based Global Detention Project, the line between immigration detention and criminal incarceration in Saudi Arabia is often unclear.

There is no independent oversight of detention practices, and the last prison visit conducted by an independent human rights organisation was by HRW in 2006. Coogle says there are at least 10 facilities, including prisons, used for immigrant detention inside Saudi Arabia, but the precise figure is not known.

“They are treated like animals in these prisons,” said one aid worker.

A Saudi official denied the allegations. An official told the Guardian that “no patient is deported until after their treatment and recovery” and that “no offender is deported by force”.

“All procedures for dealing with cases of illness, childcare, and deportation of offenders are carried out with the oversight of the diplomatic missions of their respective countries,” the official added.

Tayib said that in the first facility, where he stayed for five days, he received no food or water in the first 24 hours. When he and other inmates started complaining, some were taken outside and beaten. When food eventually arrived it consisted of only a plate of rice between six of them.

Beatings, he said, were commonplace: “Even without you saying or doing anything, they beat you. We didn’t know the reason. They punched us with their hands and when they got tired they would start kicking. It’s like a sport.”

Ethiopian migrants walk beside a road in Yemen en route to Saudi Arabia, where they hope to find work. Photograph: Susan Schulman

Other former detainees told the Guardian similar stories, as did those interviewed by HRW for a new report.

The most notorious detention facility is in Al Dayer, in southern Saudi Arabia’s Jizan region. “These are some of the worst conditions I’ve ever heard of – really the whole nine yards,” said Coogle.

Former detainees reported abuses such as chaining inmates together; cells with overflowing toilets, forcing inmates to sit in faeces and urine; food thrown through cell windows so that inmates fight over scraps; prohibiting Muslims from praying, and tearing crosses off the necklaces of Christian inmates; and an absence of clean water or sanitation.

Former detainee Abdulrahim Sofian told the Guardian that guards trod on his back as a form of punishment, and repeatedly called him and other detainees “dogs” or “animals”.

Like Tayib, his possessions, including clothes, were confiscated: “We left everything there,” he said, wearing an Ethiopian Airlines blanket as a shawl.

Saudi authorities are also accused of detaining and deporting minors, despite the country having signed and ratified the convention on the rights of the child, which indicates that children should not be detained on account of their migration status.

“Children are among the several thousands of people arriving each week from Saudi Arabia and we are particularly worried about them as they require special protection and social assistance,” said Mohamed Morchid, country director for Médecins Sans Frontières in Ethiopia.

A Saudi official denied the allegation: “Children are not deported except with their parents, and children are not subject to any penalty.

“Children whose parents are not known are kept in social care facilities with Saudi children.”

IOM figures show that 6% of those returned since March 2017 were minors, though aid workers said the prevalence of false documentation means the true figure may be higher. The Guardian spoke to two deportees who said they were 15 years old, though the laissez-passer documents (“safe conduct” passes) granted by the Saudi authorities claimed they were adults.

There are very few organisations in the capital able to take care of minors, which means the majority end up on the streets shortly after arriving back in Ethiopia.

Ethiopians awaiting repatriation are seen in a bombed-out football stadium in Aden, Yemen. Photograph: Susan Schulman

Ethiopia has drawn close to Saudi Arabia since Abiy Ahmed made the kingdom his first overseas visit – and accepted offers of financial aid – after he became prime minister last year.

“It is clear the Ethiopian government doesn’t want to make any issue with the Saudis about this,” said one aid worker.

Berhanu Aberra, director for overseas employment at Ethiopia’s ministry of labour and social affairs, said his government was working “with the respective Saudi bodies to respect and protect the dignity, rights and safety of our citizens” and to ensure all are “treated safely until back home”.

The Ethiopian government has also spoken warmly of Saudi Arabia as a partner, noting that it is paying for transport for deportees.

But aid agencies said they need more support.

The IOM has estimated that $23.8m (£19.6m) is needed to assist returnees with reintegration efforts. Last year, Saudi Arabia reportedly promised $30m to help but this has not yet materialised. The EU has refused to offer funding to assist with returns to Ethiopia from the Gulf.

Meanwhile, despite the dangers, migrants from Ethiopia and the Horn of Africa continue to make the journey to Saudi Arabia in persistently large numbers.

Danielle Botti of the Mixed Migration Centre said the number of arrivals in Yemen in April and May – at least 18,000 a month – were the highest recorded since 2006. The MMC estimates that more than 10% of these could be making repeat journeys.

“What they need is social support,” said one aid worker. “Because otherwise they go again and again.”