Yemeni police have clashed with anti-government protesters for a third day in a row, as they demanded political reform and the resignation of Ali Abdullah Saleh, the president.
Several thousand protesters, many of them university students, tried to reach the central square in the capital Sanaa on Sunday, but were pushed back by police using clubs.
Witnesses said several protesters were injured and 23 people were detained by police.
Human Rights Watch (HRW) said that the security forces had used electroshock tasers and batons against the demonstrators.
The US-based organisation called on the Yemeni government to cease all attacks against the demonstrators and investigate and prosecute those responsible for the violence.
"Without provocation, government security forces brutally beat and tasered peaceful demonstrators on the streets of Sanaa," Sarah Leah Whitson, the Middle East and North Africa director at HRW, said.
"The government needs to take full responsibility for this abuse."
Protesters told the organisation that an anti-government demonstration organised by local, independent activists had started at the new Sanaa University.
Later, dozens of pro-government activists arrived at the university and attacked the demonstrators.
To avoid a clash, the anti-government demonstrators said they then began a march.
Pro-government protesters were also seen in the city centre, at some points facing off against the opposing demonstrators.
On Saturday, clashes broke out in Sanaa between groups supporting and opposing the government after men armed with knives and sticks forced around 300 anti-government protesters to end a rally, the Reuters news agency quoted witnesses as saying.
A day earlier, celebrations in the capital over the ousting of Hosni Mubarak, the Egyptian president, turned to clashes when hundreds of men armed with knives, sticks, and assault rifles attacked the protesters as security forces stood by, according to HRW.
The 18-day uprising in Egypt has raised questions about the stability of Yemen and several other governments in the region.
Saleh has been in power for three decades and has tried to quell unrest by promising not to run for office again.