Describing the effects of gas, activist Ahmed Salah said he was still coughing blood 15 hours after being exposed to it. “I was wearing a gas mask. My eyes and mouth were covered as was my skin. As soon as the gas came people around me fell on to the ground in convulsions. I felt very weak and dizzy. I couldn't focus and I started coughing. Coughing up blood.

“People have seen three different kinds of canisters. Most are marked CS but some have seen canisters marked with the letters CR and there is a third canister that has no markings at all.” According to Salah, gas also appeared to have been pumped into the square on Tuesday evening (November 22).

In a statement put out via Twitter, Ramez Reda Moustafa, a neurologist at Cairo's Ain Shams University, described seeing cases where exposure to gas had “caused extra-pyramidal symptoms [involuntary jerks in extremities and trunk mimicking a convulsive seizure, occulogyric crisis, etc] and little respiratory distress.”

© Guardian Newspapers Limited, 2011

More In: Comment | Opinion