In Sanaa, the capital, four people were reportedly killed and nearly 100 wounded on Tuesday when riot police stopped protesters marching towards the capital's main Zubeiri street.
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Protesters threw stones at riot police and set fire to a security vehicle, witnesses said.
An Al Jazeera producer reported that four female doctors who had been helping to treat injured protesters were taken away by security forces.
Tariq Numan, a doctor working at a field hospital outside Sanaa university, told Al Jazeera that "hundreds of injured people" were being treated at his facility following the violence.