Hundreds of anti-government demonstrators clashed with supporters of Yemen's president on Monday south of the capital, with both sides hurling rocks as protests escalated in the impoverished Arabian Peninsula state.
Witnesses said police had fired shots into the air but were unable to control the crowds in the industrial town of Taiz, while in Sanaa protesters inspired by an uprising in Egypt vowed to march to police intelligence headquarters.
"Hey Ali, get out, get out," anti-government protesters shouted at Sanaa University, referring to President Ali Abdullah Saleh, a U.S. ally against al Qaeda's resurgent Yemen-based wing. "There is no solution except to leave."
Police stood between around 500 anti-government protesters and a rival group of around 100 supporters of Saleh at Sanaa University, a frequent launchpad for demonstrations, to prevent skirmishes.
Anti-government protests gained momentum in recent weeks, sometimes drawing tens of thousands of people, and the threat of further turmoil prompted Saleh to offer significant concessions, including a pledge to step down in 2013.
The protests have turned to clashes in the last four days, with rival groups beating each other with batons and fists. On Sunday, police forcibly broke up a march in the capital.
But analysts say Yemen is not yet at the point of an Egypt-style revolt, and any upheaval would likely unfold more slowly, and perhaps with more bloodshed, in a heavily armed country where tribal allegiances run strong.
Protesters in Sanaa said they were demanding the release of activists arrested over four straight days of rallies, including around 220 held in Taiz, whom the opposition said had already been freed.
Saleh, who has ruled fractious Yemen for 32 years, postponed a visit to Washington on Sunday due to conditions in the region, according to a state news agency.
Human Rights Watch criticised police for what they described as unnecessary brutality that included using electroshock tasers against demonstrators.
"Without provocation, government security forces brutally beat and tasered peaceful demonstrators on the streets of Sanaa," said Sarah Leah Whitson, the Middle East and North Africa director of the U.S.-based rights group.
Although pro- and anti-government protesters have clashed in recent days, police had generally stayed out of the fray in Sanaa, but crackdowns have been stronger outside the capital.
Yemen is struggling to quell separatist rebellion in the south and cement a truce with Shi'ite insurgents in the north at the same time it is fighting a resurgent wing of al Qaeda based in Yemen.