Security forces in Yemen have fired on anti-government protesters for a second day, killing four people and wounding several, doctors say.

Troops opened fire on tens of thousands of people marching in the capital Sanaa to call for President Ali Abdullah Saleh's resignation, witnesses say.

At least 12 people were killed and 80 people wounded in a similar protest on Saturday.

President Saleh has been battling eight months of street protests.

He has so far resisted calls from many Western countries to end his 33-year rule, despite saying on several occasions he was prepared to do so.