Witnesses said police fired shots into the air but were unable to control the crowds in the industrial town of Taiz, while in Sanaa protestors vowed to march to police intelligence headquarters.

'Hey Ali, get out, get out,' anti-government protestors shouted at Sanaa University, referring to President Ali Abdullah Saleh, a US ally against al-Qaeda's resurgent Yemen-based wing.

'There is no solution except to leave.'

Police moved between around 500 anti-government protestors and a rival group of around 100 supporters of Saleh at Sanaa University - a frequent launch pad for demonstrations - to prevent skirmishes.

Anti-government protests have gained momentum in recent weeks, sometimes drawing tens of thousands of people, and the threat of further turmoil prompted Mr 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. Yesterday, 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.

Those 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.

Human Rights Watch criticised police for what it 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 rights group.

Although pro and anti-government protestors have clashed in recent days, police had generally stayed out of the fray in Sanaa, but crackdowns have been stronger outside the capital.