Doctors in Yemen have told the BBC that unidentified gunmen fired on an anti-government rally in the capital, Sanaa.
The gunmen fired from rooftops overlooking the central square, which protesters have named Taghyir (Arabic for "change") Square.
President Ali Abdullah Saleh declared a national state of emergency but denied his forces were behind the shooting. A month of violence has shaken Yemen, with protesters demanding that the president step down.
People in Sanaa have been telling their stories to the BBC:
Live ammunition was fired.
I made a plea a short time ago for help to the field hospital because our capacity is far below the casualty numbers. And the numbers are still growing as we are still receiving casualties.
Many of the wounded are in a critical condition.
We have no clear statstics now, as we are busy saving lives. The injuries are mostly in the head and chest but there are also injuries all over the body.
We have cases targeted randomly and others were clearly shot to be killed.
Most of those killed were shot in the head and chest.
What is happening now is a crime in all possible terms.
Today after Friday prayers, the police attacked protesters, firing from houses around the square.
They were dressed as civilians. Some of these houses have now been taken over by the protesters.
They killed many people - more than 40. I didn't see it with my own eyes but I am now here in the square where there are injured people and people still being taken to hospital.
One of the gates was attacked by the police.
It is calm here now. There are protesters at the gate protecting us.
We heard the shooting here today as I live near the university but it wasn't until I read the internet story that I knew exactly what had happened.
After the shooting I went to the pro-government camp and then into the city.
In the camp everyone was sitting around - they just seemed to be really lethargic and there was minimum security around the camp.
I think it was clear that the security forces were getting ready for some kind of action against the protesters because they were constructing walls to block off the anti-government camp near Sanaa University after last week's crackdown.
I have also been to the anti-government camp in the last week. They have security lines that you go through when you enter the camp. It is strict and you don't take weapons in.
I did notice two days ago that they have taken to wearing hard helmets but there is no visible evidence of people having weapons there.