Tens of thousands of Yemenis turned out for protests against Ali Abdullah Saleh on Monday [Reuters]

Government forces have fired artillery shells in the Yemeni city of Taiz, where gunmen armed with automatic machine guns attacked protesters in the city's main square.

Sources told Al Jazeera on late on Monday that heavy artillery shells hit the northern part of the city.

A security official told the Associated Press news agency that one person had been killed and three others wounded in the attack by Republican Guard forces. The official spoke on condition of anonymity. He said the forces had targeted armed tribesmen.

Mahmoud Taha, an activist in the city, said that the shelling had hit several districts and that armed tribesmen had responded by attacking several army vehicles.

Taiz has been a focal point of anti-government protests in the city, with tribesmen attacking government troops in the city frequently over the past few weeks.

Hundreds of thousands of anti-Saleh protesters rallied in Sanaa, the Yemeni capital, and other major cities earlier in the day.

Protest leaders told tens of thousands of people in Sanaa that a proposed transition plan that envisages Saleh handing over power within 30 days to his vice-president, who would form a national unity government and hold elections, would only prolong the crisis.

They said that they wanted a faster pace of change.

Meanwhile, a government official said on Sunday that some 54,000 Yemenis had fled the southern province of Abyan since fighters took over its capital last month. He told a visiting UN delegation that the refugees had fled to neighbouring Aden, a port near a strategic sea lane.