The president of Yemen has imposed a state of emergency across the country.
The announcement by Ali Abdullah Saleh came after armed men opened fire on a big anti-government demonstration in the capital Sanaa, killing at least 39 people.
About 200 people have been wounded in what appears to be the worst day of violence since protests began a month ago, the BBC reports.
Medical sources in Sanaa say most of those killed or injured were shot in the head or chest.
The state of emergency means civilians are not allowed to carry arms.
The president has denied his forces were behind the shooting.