Government supporters wielding knives and handguns attacked protesters in southern Yemen, leaving one dead in the latest in weeks of demonstrations demanding the president step down.
In a separate development in the increasingly chaotic Middle Eastern nation, suspected al Qaida gunmen killed four soldiers from the elite Republican Guard forces in a mountainous region.
Even before Yemen was hit by the wave of protests that began in mid-February, the country's government was weak and struggling to contain one of the world's most active al Qaida branches, a secessionist rebellion in the south and a Shiite uprising in the north.
Then protesters caught a wave of unrest that is sweeping many Arab nations, calling for President Ali Abdullah Saleh to step down after 32 years in power, a demand he has repeatedly rejected while trying to assuage opposition groups.
Mr Saleh has said he would not seek another term in office in 2013, and offered to form a unity government with opposition figures, but the overtures have failed to appease the protesters.
Eyewitnesses said that "government thugs" marched toward the protesters in the southern province of Ibb without being stopped by security forces. They spoke on the condition of anonymity for fear of government reprisal.
One protester was killed and 37 others were injured in an attack that took place on the main square, where protesters have been camping for a week.
The clashes came after some 200,000 protesters in Ibb, 120 miles south of Sanaa, took part in a demonstration on Friday calling on Mr Saleh to step down.
Meanwhile, security officials said the suspected al Qaida gunmen ambushed the soldiers while travelling in a vehicle to distribute food to fellow troops manning checkpoints. They said the assailants managed to flee.