Yemeni medics and witnesses say security forces have opened fire on an anti-government protest in the capital, Sana'a, killing at least six demonstrators.

The protesters were marching from their base in Change Square toward government buildings when gunmen loyal to President Ali Abdullah Saleh attacked them on Tuesday. At least 50 demonstrators were wounded.

More than 30 people have been killed in Yemen since Saturday in government crackdowns on the protest movement and battles between pro-Saleh and anti-Saleh forces.

The opposition activists have been protesting for an immediate end to Mr. Saleh's 33-year autocratic rule for the past 10 months. He has repeatedly rejected a Gulf Cooperation Council proposal for him to step down in return for immunity from prosecution.

Mr. Saleh says he will not hand power to long-time opposition rivals who he accuses of seeking to destroy the country.