Security forces in Yemen have fired on anti-government protesters during a march in Sanaa, killing at least 26 people, witnesses and doctors say. More than 300 people have been treated for gunshot wounds, at least 25 of whom are said to be critically hurt. Security forces opened fire with heavy weapons as tens of thousands of protesters called for President Ali Abdullah Saleh to step down on Sunday. Witnesses said gunfire continued in areas of the capital overnight. "There was gunfire throughout the night, and explosions, and I'm actually close to the protests and I can still hear gunfire," Laura Kasinof, a correspondent for the New York Times in Sanaa, told the BBC. The clashes are the deadliest in Yemen for several months. The defence ministry said the trouble began when protesters threw petrol bombs at the security forces. In a statement posted on its website, the ministry said an Islamist group had opened fire on the protesters. But protesters dismissed the government's version of events, saying their rally had been peaceful. Military defections Witnesses said the security forces opened fire on the protesters after they left an encampment in Sanaa and marched towards the presidential palace. There are reports that machine guns and even anti-aircraft weapons were turned on the crowd. Tear gas and water cannons were also deployed, said witnesses. The confrontation then developed into running street battles. One protester told Reuters news agency he had seen his cousin shot dead. "We protested and camped here together since the beginning," said 23-year-old Tawfiq. "I turned around and saw him fall. I tried to hold together the wound. I can't bring myself to call his parents." Doctors at hospitals in the capital said 26 people were killed. Hundreds of people were being treated for gunshot wounds and teargas inhalation. The violence marks a sharp escalation in the stand-off between protesters calling for Mr Saleh to step down, and security forces loyal to the president. Some units of the military have defected to the opposition. There were reports of exchanges of gunfire between the two forces although a spokesman for Maj Gen Ali al-Ahmar, who defected several months ago, denied it, Associated Press news agency said. Mr Saleh is in Saudi Arabia recovering from a bomb attack on him in June. There have been widespread protests against his 30-year rule.