Tens of thousands of anti-government protesters in Yemen hurled stones at riot police backed by tanks in the southern province of Aden as rival rallies were held in the capital.
Nearly two months of daily protests demanding the removal of President Ali Abdullah Saleh have so far failed to bring an end to his 32-year rule over Yemen, a corner of the Arabian peninsula beset by poverty, conflict and a long list of other problems.
Seizing on the example of Egypt and other uprisings, the protesters want to open up the country's political system as a path toward a better life.
President Saleh's crackdown has been so harsh that it has propelled some of his key allies - even his own tribe - into the opposition camp. Nevertheless, he refuses to step down immediately, saying the country would sink into chaos without an orderly, negotiated transition.
Reflecting the stand-off, both sides have held demonstrations in the capital, Sanaa.
Mr Saleh appeared briefly at the start of the pro-government rally, pledging to sacrifice his "blood and soul" for the sake of the Yemeni people.
In Aden, protesters set tyres on fire and built barricades with large rocks at the entrance of main roads to prevent tanks from moving. Stone-throwing clashes broke out with police.
Yemen's unrest is of concern to the United States because the country is home to al Qaida's most active franchise.
US State Department spokesman Mark Toner said Washington was concerned about the situation in Yemen but insisted counter-terrorism co-operation was continuing between the two countries.
The US gives Mr Saleh's government financial and military aid for fighting al Qaida and has provided training to some of his forces.