Tens of thousands of protesters calling for President Ali Abdullah Saleh to step down were marching on the presidential palace. 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.

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 automatic weapons and even anti-aircraft guns were turned on the crowd. Tear gas and water cannons were also deployed, said witnesses. The confrontation then developed into running street battles.

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 standoff between protesters calling for President Saleh to step down, and security forces loyal to the president.