Troops loyal to President Ali Abdullah Saleh of Yemen have killed at least 16 people in in the capital Sanaa.
Anti-government protesters say they came under attack from guns and mortars.
The clashes happened a day after fighting between forces loyal to Mr Saleh and armed tribesmen.
The president returned home suddenly and unexpectedly on Friday after spending three months in Saudi Arabia recoving from an assassination attempt in June.
Protests against the rule of Mr Saleh began in January. He has been in power since 1978.