The teargas used by interior ministry troops in Cairo's Tahrir Square is supplied by a US company. Demonstrators say cartridges retrieved from the scene are branded with the name and address of Combined Systems Inc (CSI).

The firm is located in Jamestown, Pennsylvania. It specialises in supplying what it calls "crowd control devices" to armies and "homeland security agencies" around the world. It also manufactures lethal military equipment.

Protesters say the CS gas seems more powerful than that used by Egyptian police during the country's last popular uprising in February. "It's stronger, it burns your face, it makes you feel like your whole body is seizing up," one witness said. He added: "It doesn't seem to be combated by Coke or vinegar."

Experts told the Guardian the gas was likely to be standard CS gas, but the effects could be exacerbated by physical exertion.

As well as the effects of the teargas, protesters have suffered grave injuries to their heads and faces from rubber bullets. There are also reports of live ammunition being used. Dozens of people have been taken to makeshift hospitals after inhaling the choking gas fired by the Central Security Forces.

The export of teargas to foreign law enforcement agencies is not prohibited. CSI has also sold teargas to the Israeli police, where it has been deployed against Palestinian demonstrators, as well as, reportedly, to the regime of Tunisia's ousted dictator Zine El Abidine Ben Ali. Nevertheless, the revelation that people are being gassed and hurt by US-manufactured projectiles is embarrassing for the Obama administration.

"We have seen the illegitimate and indiscriminate use of teargas," Heba Morayef, a researcher with Human Rights Watch in Cairo, said, of Egypt's most recent street protests, as well as the original revolution in February. "There are a few cartridges from Italy but the vast majority are from the USA."

She said teargas did not constitute direct military aid, since it was sold to the interior ministry rather than the army. But she added: "Ideally governments should be verifying who they are selling teargas to."

Morayef said the gas was having a devastating effect on its victims, with everyone left choking, and hundreds forced to seek medical treatment. Protesters have also retrieved 12mm rubber bullet cartridges made in Italy. "One person I know ended up coughing up blood," she said. Human Rights Watch intended to examine the canisters to discover exactly what kind of gas was being used, she added.

Alastair Hay, professor of environmental toxicology at Leeds University, said police in Cairo were almost certainly using conventional CS gas. "It's a standard riot control agent which has been around for a very long time," he said.

Hay said its effects were extremely unpleasant. "It's an eye and respiratory tract irritant, largely. It will also cause skin irritation."

The chemical compound used in CS gas – 2-chlorobenzalmalononitrile – was "perfectly legitimate", with many commercial companies involved in selling it, and domestic governments willing to make use of it in riot situations, he added.

US army trials showed CS gas had a far more serious effect on people taking part in physical activity than those sitting passively, sometimes leaving its victims needing intensive care afterwards. The way to get rid of it was "constant irrigation" to wash away the affected areas, Hay said.

There was no immediate comment from CSI.

The company's website says it was founded in 1981. It adds: "Combined Systems Inc (CSI) is a US-based firm that supports military forces and law enforcement agencies around the world. CSI is a premier engineering, manufacturing and supply company of tactical munitions and crowd control devices globally to armed forces, law enforcement, corrections and homeland security agencies.

"[…] In addition to its military products, CSI markets its innovative line of less lethal munitions, tactical munitions and crowd-control products to domestic law enforcement agencies under its law enforcement brand name, CTS. CSI also supports its wide base of international military and law enforcement customers with its line of non-lethal munitions."